If you convert an AI opponent's cities, will they decide not to found a religion?
I noticed that if you convert AI cities to your religion, they still try to found their own religion. If your religious pressure is really high tho the AI religion has a really tough stand and probably won't be any danger to your own. I have also noticed in a few games that the AI still tries to spread their religion even when your religion is dominant.
So my answer would be "No you cannot stop the AI from founding a religion" But if you have a broad base for your religion even in their cities that is not really a big problem.