You can attack, no problem at all. Just be sure not to raze the cities. And when you want to eliminate them, you are sure to get a rep hit. Mostly this is not serious enough for you to lose a vote, if you stayed decent the rest of the game.
However, you can do anything you like with a civ as long as that civ is not known to the rest of the world (no communications). So you can easily get rid of 1 or 2 civs in the beginning of the game.
Satchel already pointed you to his great thread...