Help - Leaking transmission fluid

I have a Hyundai Elantra GT 2002, with about 90k miles on it. It is an automatic transmission, 2 wheel drive. I recently had the transmission fluid changed at the recommendation of the mechanic. Since then, my car has been leaking transmission fluid. The mechanics keep claiming it is residual. I have been to them several times, and all they did was wipe the leaky area. It seems they are getting tired of me, because the last call I made, they said it must have been leaking before they even touched it. This is highly unlikely, as I would have definatly noticed
any stains on the driveway. What do I do now? I am going to make another trip there soon, but I doubt that will have any effect. It looks like my only option is to accept it was leaking before and pay them to fix their own mistake.