You kill them and they build on their martyr's blood. Get them to water down their faith, and a little group somewhere will rediscover the real faith and they'll start over again - they have an infuriating way of regenerating themselves. And these Christians know how to turn a negative into a positive. They turn our best laid plans upside down. Get a couple of their famous religious figures to commit adultery or visit a prostitute, and they'll simply produce a thousand seminars and books on sexual fidelity, and the net effect will be greater morality among many of them, not lesser.
Denominations are, of course, good targets.
However, as quickly as one cools off, they'll start a new one. These Christians produce new denominations faster than roaches reproduce baby roaches. Same with local churches. No sooner than we get a local church to die spiritually and there'll be two brand new ones cropping up in some school auditorium across town. A few times in history we've pretty well got the whole church to go lukewarm.