How can you keep your ex off of crystal meth if you still love her?

Actually freewill is a great thing and locking someone in a room for any length of time is not only cruel but also a felony. Noone will change by force or coercion.

"FREE WILL IS A BAD THING!!! You can save her if you really want to. Invite her to your house, tell her you want to get back together with her, put a 6 week supply of food in your closet.. maybe canned foods or whatever, remove any sharp objects and then lock her in your bedroom for 6 weeks and don't let her come out. I read about a guy who did that and they were addiction free afterwards. Bolt the door with five or six locks because there is going to be a lot of resistance... and remove any thing that might assist her in suicide."

"After that, call her crystal meth dealer from a payphone and tell him the police are looking for her."

Sorry, but you cannot. The only person capable of that is her. That's the thing about addictions; they are personal and each addict has to fight their own battles. It doesn't matter how many lives the addiction destroys it still belongs to the addict. Love her, pray for her, support her in her battles, but protect yourself foremost. I know a little something about addictions from being on both sides, and neither side is a good place to be.