Depending on the keg post type/style, you could need to get a different sized wrench than you currently have. If you have a decent socket set, you could have what you need already (need a deep socket though). Or you'll need to get a new wrench.
IF you have trouble removing the post, get a length of black pipe (3/4" pipe, 12" long did it for me) and slip it over the ratchet handle. That should give you the extra leverage you need to remove the post.
I would also advise, in the future, pressure testing the kegs before you fill them. Unless you get them brand new, and even then it's not a bad idea to give them a bit of a test before you go to fill them.