steam
Steam limits the number of friends you can have, but I am uncertain if there is a limit on how many people you can block.
I tried to google it, but information I found was conflicting. Some said it was infinite, others said it was 500, again others said it depended on the Steam Level. However, none of those were from official sources and merely sounded like best guesses.
So is there a limit on how many people you can block on Steam?
You cannot install a Steam game offline. It needs to connect to the Steam servers the very first time to confirm you own the game.
If you do not want to redownload the game, go online, click install, let it go as far as where it starts downloading, close Steam completely and copy the files from the SteamApps folder on another machine, then launch Steam again. However, this process still requires an internet connection on first play.
Xbox LIVE has this feature, but I don't believe that Steam does.
Instead of reporting them, you can block communication with them.
(Apologies to Retrosaur, I just had his chat window open still :P)
First, click the arrow next to their name (which I have highlighted with a red box), which will drop down a menu.
Click the "Block All Communication..." option, and you'll see the dialog on the right.
Once blocked, they will not be able to send you further messages, game invites, trade requests, etc.
Best Answer
An official response from Steam Support: