When the AI i generated you will proberbly have to walk it over and see where it did not do such a fine job (close to corner of wall for example), you CAN paint where you don't want the players to be able to walk, or visa versa.
It's done in the AI menu, but don't ask me how, because I am still fumbling around with it

. I did manage to do it, though I'm can't remember exacly how I did it... I hope this will be more clear on the WIKI soon.