Curious as to how my adult Atheist son handles this I asked.
He said that he says "Please, spare me your Sorceries, I have no time for Sorcery."

It would depend on whether the prayer was for my salvation or the result of a blood test. Two VERY different responses.

I say, "Thank you."

"Please don't pray to your Blood God for me."
Is how I often respond
Is is a cartharsis
It shocks the system of the believer. They are either silent that I called Jesus a Blood God
OR they begin to insist it is not a Blood God, when it clearly is a Blood God and this can be easily pointed out . . .down the rabbit hole
I get few repeat offenders.

I was the only one who gave a snitty comment, sorry, I am in a snitty mood today.

I hate it because ot makes you into a poor wee thing that needs praying for, and I need that response like I need a hole in the head - given that my society is now completely down the tubes I would probably want to say something smartarse like - 'get lost!'

I say "If that's what you want to do.", I don't want to give them the impression I approve of prayer to sky daddy. That might sound gruff but I have my reasons.

Sorry I got sidetracked, my response is always, no thanks, save it for someone who would appreciate it.

No good, I like the other fella

Gets them thinking.. LOL

Swiftly moving on.

If they are genuinely trying to be nice or show empathy as in a loss of loved one I say "thank you".

If they are saying it in order to degrade my beliefs I say "no thanks"

Allah willing? Nothing fails like prayer?