The strongest player in the game is a close ally of mine and we each founded and follow of one of the strongest two religions in the game, with little cross-over between religions within our territories.
I've just spotted that he has a Great Prophet approaching my territory. According to the Civopedia they are powerful at spreading religion, and I'd rather not have a huge hit to the followers of my religion in my cities.
How can I prevent another players Great Prophet from damaging the religion in my cities, preferably without causing a major diplomatic incident.