Bow your head an loudly pray to Odin for strength in battle and the power to smite your enemies. Smile and walk away.

That's a very British response and I feel for you. Perhaps some American-style pentecostals have taken up near you? If so, please accept my apologies as an American for yet another of our unsavory exports.

I like @jeshuey's idea of quoting Matthew 6:5 in response best of all the prior responses.

Point them in the direction of the nearest church.

I “pray” out loud when I’m at the gym and I don’t want to hear other people’s loud conversations or the TV is annoying me.

Do not hesitate to point out to such folks Jesus' words in Matthew 6:5 -- "And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward."

My mother used to scream at us that praying silently in your own head was a sin, you pray out loud or not at all, praying silently meant you had something to hide.

I live in the Bible Belt and have come across this in a private community pool. I've been asked if I want to pray with them. I say "no" and just continue to enjoy my pool time. They're not screaming so it just sounds like chatter to me. I have never experienced any thing like that in 52 years in New England.

I've been seeing it for ages. It always makes me really uncomfortable.

I live in the bible belt here in the U.S. and see this quite often when I go out to eat. I ignore them.