You wouldnt earn achievements from a weekend free game, right?

you do.

You do, the thing you don't get is get cards, for that you have to purchase the game

create another Steam account only for playing free-weekend games, easy solution for people with OCD like you (and like me)

There is no other way to disable achievements than by simply not to play the game.

SAM.

afaik they don´t count to your rate, because you don´t own the game.
they will still be there when you buy the game, but won´t matter if not.

I'm not sure if it'll work in this case, but in a lot of games you don't get achievements if you play in offline mode

I earned an achievement for Borderlands 2 during the free weekend and that achievement isn't showing up in my account anymore as it is not attached to my library.

Just use SAM to erase your stats after the game is gone.