Can Steam undo game activation?

For example if you get a cut version of a game and want to buy the uncut one.

Hi, noticed that you added me (along with 20+ other people...)

Err, why do you want to add me? :S

It's for when you add a non steam game to your inventory, like League of Legends, and want it removed from your library.

Low violence versions of games are an entirely different entry, so you can activate both. I have two copies of Sin Episodes: Emergence on my list. :v

I have a DE version of Killing Floor (less gore, low violence etc). So I can write to Steam support and ask them if they could change it to 'normal' version?

Once I bought a game and activated by mistake (He said the CD-Key was invalid so I tried it out and it activated on my account). I later made a support ticket and the Valve/Steam employes removed the game from my library and gave copy of the game to my friend. So if something similar happened in your situation they are able to move game between accounts, but only if they were accidental like mine I'm assuming.

You definitely can buy a game and then sell it to a friend, but without this activate/remove thing.

You're just jelly you don't have teeth. Most worms don't.

If it's within a reasonable amount of time (say you buy a game and it turns out to be totally incompatible with your computer because it just hates you), you file a support ticket with Steam support, ask for a refund, and outline your reasons (for example, it won't install, and thread whatever blah in the Steam forums says this is a known issue, blah de blah, so you can't play it at all). Then you hope they accept your reason and issue you the refund.

That's how.