In-Game
It wasn't wrong of you to not force the party to take him, but it did make things a bit harder for you. Without more information on what exactly is going on in your campaign, it's hard to say how exactly you should fold this new guy into the group, but I'll give you a few ideas. You're going to need a few things:
- Get the party to need the new guy.
- Get the new guy to need the party.
Let's imagine the party is off searching for the lost Book of Dragon Stuff, while the new character is back in Bigold City doing something boring -- let's say he's practicing law, but he wishes he were a wealthy merchant instead.
(At this point we've established the party's goal, the new guy's goal, and what everyone is doing.)
The new guy, as a lawyer, knows the local magistrate, who turns out to be a collector of bits of Dragon Stuff, and would love to have a chance to read the Book.
Meanwhile, the party finds out that some Dragon Stuff artifact that they want got taken to Bigold City. The party heads to town and runs into the new guy. The new guy introduces them to the magistrate.
The magistrate tells the party that the item they're looking for was owned by some nobleman who died, and they won't be able to get a look at it until his sons are done arguing over the will. If only we had someone who knew about inheritance law to sort things out!
Now the party needs the new guy to help them out, the new guy has a chance to join in with the party on their quest, and everyone involved would benefit from making friends with the magistrate, which could be done by finding the Book and letting him study it.
Out-of-Game
The methods above are ways to get the in-game party to incorporate the new in-game character.
Equally important is getting the out-of-game party together. Talk to the players sitting around the table. If they're interested in having a new player join, then you might try establishing a rule about adding new characters. Here are a few suggestions:
- All characters are part of the same party, no matter what.
- All characters are part of the same party, unless they become evil, in which case that character is retired (or becomes an NPC) and the player makes a new character.
- New characters must be approved by the party.
- Characters are made jointly by the group.
In our group, we've been using a explicit we're all the same party rule for years now, and it's worked well for us. When a new player joins, we do some in-game stuff to incorporate their character, but we also have a rule out-of-game that the new player is part of the same group.
No, it's not fair to punish a player in this way. But it's not okay for this player to take an unfair share of control over the game, either.
You are the DM — you are in control. You are in the Big Chair, and the group put you there to be responsible for pacing and mediation. You have that job because players striving for the benefit of their PC is fun but usually incompatible with players having responsibility for fair pacing of action. You don't need to resort to retributive actions like that. You need to exercise that pacing authority instead.
Don't punish — be proactively assertive
Punishing a player like this only makes "sense" if you're not in control of the game world and play pacing, but to be able to punish a player in-game requires being in control. See the contradiction? No control — in control.
Instead of letting the player take control, and then punishing them for taking it, reassert that they don't get to inject things whenever they want.
Player: I throw it at the wall... Ack! I rolled a one!
GM: You hear a cracking sound—
Player: I roll out of the way!
GM: Hang on there Speedy, I'm not done. Ahem. You hear a cracking sound, and the wall collapses onto you. Roll an Escape Save† to see how well or badly you're hit by falling masonry.
Player: But I said I got out of the way!
GM: Being good at interrupting me when I'm talking doesn't magically make your character faster in the game world. Make the roll.
† I just made that up. I'm not assuming exactly how your homebrew would work here — maybe they'd just take damage directly, or maybe something else might happen like using luck points or something — but I'm making this up for the sake of the example's narrative flow. Adjust it as you read to fit your actual game rules.
See how that works? Be confident, don't let the interruption startle you, and do not accept the contribution that is made inappropriately. In short, do not allow "instant actioning." It's rude, privileges people who are good at talking fast and startlingly, encourages being rude to get an unfair advantage compared to the other players, and adds things to the unfolding story that make no sense in-world. Just don't permit it.
When a player attempts to take control over the narrative, just ignore it. Void it, veto it, undo it — however you want to think about it, just do not dignify it by accepting and incorporating it into the "Shared Imaginary Space" — the shared and agreed on story of what is actually happening with these characters. Don't let one interrupting player take inappropriate priority over what's happening.
Remind yourself that nothing passes that doesn't gain acceptance. Remember that the other players are looking to you as the example for what is accepted, moment to moment, so you do have this authority, even if it's uncomfortable to feel like you have that much say over the narrative. Being the GM is also about leadership, and leading by example.
Calmly, and confidently, disregard the interruption and continue as if it didn't happen in the game — because it didn't yet.
Moving forward
Normally I would suggest retconning the situation, but I won't here because your player took the PC's death well. That shows me that you and your group have an high degree of mutual trust. That means that rolling back and redoing this isn't really necessary to patch things up — there's no unmet need that would justify the significant disruption a retcon can cause.
What I would do here, is talk about "instant actioning" at the beginning of the next session. Say that it's become a problem, and you don't like how you dealt with it last time. Thank the player for being such a good sport, and apologise that how you handled it lead to their character's death. Then just let the group know that, to deal with the unwanted phenomenon of instant actioning, you're no longer going to allow "talking faster than me" to interrupt what's happening in game. Since habits are hard to break, tell them that it's OK if they forget and try an instant action, and that you'll just calmly ask them to hold on until you're done.
Best Answer
Surely you mean punishing the character not the player?
The character should have a whole lot of horrible things happen to them. Wake up in a gimp suit? Be kept as "blood on tape"? Raped(1)? At least wake up naked covered in marker pen with inappropriate (and bad) drawing on his face. Certainly, the character should have been robbed of all they had on them.
Maybe that's what the player wants to happen. Are they going for a slow but sure decent into despair? Trying to gain whatever little control they have since the curse is playing havoc with their free will? ...
Alternatively, do have a talk with the player asking them about it. Depending on their reactions, you can either kick them out of the game, let it slide as a "whoops, wasn't I lucky", or commence with the poor character's descent into hell.
(1) Rape is a horrible, violent, and vile thing. It should be reported to the Police if after the fact. Each and every one of us should step in to stop and prevent it, if we ever have the misfortune of being around the event. Most rapist are well known to the victim, this is why "No Means No" is an important message. I hope this state my position on the issue clears: it is a wrong, despicable, vile, and horrible act. However, I have dealt with rape, racisms, mass murder, sociopathy, psychopathy, insanity, torture, paedophilia, xenocide, genocide, and many other horrible things in more than one game. Note that if any of my players objected to a theme, it was dropped without much ado. There is always a full disclosure of themes before I run a game. Some of my players were affected by the above themes and agreed to have them run within my game.