Sad to say you can't get rid of religious zealots, just like you can never get rid of NATS. They're like cockroaches! They continue to breed and multiply.
The best thing to do is to ignore them. As long as someone is listening, they are given attention.
By simply turning your back, it makes a bigger impact than standing in front of them and taking their abuse.
Admittedly that's harder to do at the registers, but you can always call forward the next customer.