Huh I guess this guy ¯\_(ツ)_/¯ doesn't need three '\'s in a poll question 🙄

so to say after the promotion is over we can give the key away here?

why not just let them in the list, but when selected, show that it is currently not possibly to make a GA for it

Sadly i don´t believe it´s a matter of cognition rather than a matter of mindset ..

It certainly needs something; there has been a flood of "Season Pass" or "Complete" versions of Sanctum 2 listed. I assume many are out of ignorance/confusion rather than a deliberate attempt to deceive.

The problem is:
If you use a generic message, they usually won't read it. People who are apt at reading normally don't get into these situations in the first place.
If you use anything but a generic message, you need to manually flag the game as free. The problem in the first place is that the games are being given away until they are flagged (once they are, they are disabled anyway). So this would suffer from the exact same problem, namely the lack of moderator resources to monitor games the instant they become free.
Also, if people aren't even capable of picking the correct game, even a flagging won't trigger if they select an entirely different game. Again, a generic message would probably be the only option here.

The only real possibility is to enforce clearer rules upon joining the site - maybe some quiz that you have to pass before you're allowed actual entry, proving you've at least been capable of reading and understanding.

Don't remove the game from the list in the first place, but make it so when it's selected, the user is notified that they can't make a giveaway for it as it is currently being given away for free elsewhere.

^ This.

Personally, I think it is foolish to remove the game from the list, which always led to false giveaways. And nowadays they neither affect CV or point generation, and it is easy enough to hide any game you don't want to see, so it begs the question why is this (still) being done?

ʰᵘᵐᵇˡᵉ ˡⁱᵗᵗˡᵉ ˢᵉˡᶠ ᵇᵘᵐᵖ ᵇᵉᶜᵃᵘˢᵉ ˣᶜᵒᵐ
☺

The second idea is the best, cause they will know that the game is given away.
Still, people are intentionaly makin'g ga's for free games.

while im willing to give the benefit of doubt, me thinks many of those people purposefully exploit the system anyway and thus warnings wont have any effect on them.

ᶜᵒᵐᵖᵃⁿʸ ᵒᶠ ᵇᵘᵐᵖ

Go with the second option. Leave the games on the list, and when they pick a disallowed game, throw up a warning that the game is currently not allowed, and include language about not using an alternative title on penalty of banhammer.

Option 2 sounds very good