Permanently removing a game from account and SG stats.

Well, if you delete something that you've won, I would guess it would show up as unactivated win.

I would test with a separate account and some freebie/bundle trash first

Zumzoom it will see it as unactivated wins.
A few months back there was one guy who wanted to stop using SG and used that system to get a permanent suspension

Replying top level for OP's visibility:

This is what will happen - you will have non-activated wins, and you may be subject to suspension as a result.

My personal suggestion on it is simply to hide games you don't want to see in your Steam library.

You're only other option is to ask the people who's giveaway you won to delete the giveaway but i'm guessing that will be a lot. They will probably want to be recompensated with another copy of the giveaway for them to agree to this but there is no harm in asking I suppose. Best of luck whichever way you go with this. As Gaffi has said, the easiest way is to hide it from your steam library.

If anyone don't know how to hide games in steam: how to hide games in your steam library or if you prefer a video guide

Alternative option - If I recall correctly, you may provide a functioning key/copy to the giveaway creator and ask them to request a re-roll. Of course, it would be a personal arrangement between you and the giveaway creator, so not all of them may agree.

Also I'm not 100% sure you can do this, so if any support member wants to chime in and deny/confirm my suggestion, go ahead.

This is what happen to me when I realized my library has reached 4 digits
all those trash game from cheap bundle
why Do I even struggle to collect game anyway

Searching : How to delete Dota 2 Playtime record

Makes one wonder why would anyone add trash they to steam lib in first place... Not to mention enter sg ga for it.

No wait, cards...

Personally, every time I activate a game I won here, I add it to a specific library category so to be sure I never remove it in case I forget where I got it from.