I say "thank you" and leave...

If they're being 'aggressively religious' then I feel they have, themselves, almost 'declared war' - in that circumstance I don't feel THEIR feelings deserve to be 'protected'. How I respond will depend on me, and how I want to respond.

If, however, they are NOT being 'aggressive' then I don't have any desire to cause them distress.

I just reply "Thank you" and leave it at that. I do the same thing when someone says "god bless you" when I've done something they're grateful for. Really no point in being a dickhead atheist over such meaningless expressions.

I ignore them.

"Your time to waste bucko"
Or
"I'll ask drop dead Fred to watch over you in return"
Or
"Ah prayer, the most sanctimonious, vainglorious way of doing fuck all ever invented. knock yourself out."

If you don't care about them, you could say, "Anything that keeps you off the streets."

I ignore the everyday lalaland prayer unless the situation demands something tangible...

The miracle of prayer, the only person guaranteed to feel better, the one who is praying...

Don't waste your time as an atheist I am going to hell generally stuns them enough to quit.

Not very well