As of the latest update to Steam, you can now PERMANENTLY remove games from your Steam account!

Steps

1. Log into Steam Support
2. Select Games, Software, etc.,
3. Select the game from the search function that shows up
4. Select the option: I want to permanently remove this game from my account.
5. Select the option: Ok, remove the listed games from my account permanently.
6. Repeat as many times as you have games you'd like to get rid of.

You can browse over your entire library and choose which software you'd like to permanently get rid of. But note, once you have removed a game from your library, it's gone permanently, so if later down the road you'd like it back you'll have to purchase it again.

You'll also need to watch out for some other issues. The thread linked above has noted that if the game is part of a bundle (HumbleBundle, etc.), removing one game from a "package" may require you to remove all the games from that specific bundle.

I'm afraid there's no way to opt out of these recipt emails from Steam. However, if you use Gmail, you can create a filter.

Click ⚙ > Settings

Filters tab > Create a new filter

Put noreply@steampowered.com in the From field and You have sold an item on the Community Market in the Subject field then click Create filter with this search

Finally, if you wish to archive but not delete it, check Skip the Inbox (Archive it) and then click Create Filter