Don't hold a grudge against the AI religious spam. They are helping your game! Just don't panic. Have you tried the strategy I suggested? In the beginning at least, there is no good reason to protect your capital (or your other cities). Sure, you want your religion in your cites, and you want to buy your faith buildings sooner than later. But so long as you don't mind waiting until the Renaissance, none of that should be a problem!
You cannot purchase inquisitors until you enhance, but they are not really a good value anyway. There is no reason to protect your cap.
3rd GPr is just 500 faith. A GPr spread acts like an inquisitor + missionary. So each GPr == 4x inquistors + 2x missionaries.
Build the Grand Temple, your Holy City will convert back soon enough.
Inquisitors on station can be useful -- but only after your core cities are up and following your religion and you still facing waves of foreign GPr.
Buying inquisitors early is not a good use of faith. Unless very early (or buffed by Reformation belief) missionaries are all but useless. Fighting off spammers is losing battle. Buy all the faith buildings you can, but otherwise save faith instead for a GPr.
If things are really crazy, once you get your GPr, sit on it a while until you have an inquisitor on deck too.