If you don't believe in God, and by extension believe religion to be a powerless shell of a sham, surely that would be the end of the matter. Excuse us theists for pointing out the silly superstitious nature of atheists being de-baptised.
It's a bit like an adult sending a letter to Santa telling him he doesn't believe in him anymore.
If you're worried about a church having you on the membership list, just write a letter asking to be removed.