A less drastic, but equally annoying solution might be to just turn it off for a month. See what they bill you then.
"It was turned off" is a lot more likely to persuade a small claims court to your side than "I was overcharged by 14%, and here are the dozen esoteric ways I can prove it".
How do you as a consumer prove conclusively it was turned off when AT&T will say "Nuh uh. He was using it the whole time!". Then it just gets into "He said/She said" territory and I can assure you that judges hate that kind of stuff. Plus, we had another poster make the good point that even if you try to go after AT&T, they'll use their army of highly paid lawyers to argue "Change of venue" or try to show how your contract allows them to get summary dismissal. A lot of companies now have clauses that state that you agree to arbitration, which means you can't go to small claims court against them. And they can ask for the arbitration in a location as far away from you as possible so unless you're insane and willing to spend thousands of dollars to maybe win a few hundred back, it's just not worth it. I am having a hard time understanding how this is a constant problem for the original poster though unless he's trying to run a business via what is supposed to be a residential connection or he's the king of torrents. I've had AT&T for years and I've never exceeded my limits even once. Going to Uverse, as suggested in another post, is also a good idea as AT&T is known to deliberately degrade their DSL service anyway. Uverse is so much better than the old DSL service they had, I really don't think it's rational to stay with DSL if you can possibly get Uverse where you live.