I'm going to tell my boyfriend's ex that we're together now.
Why are you going out of your way to tell his ex that?
I want her to know he's off limits.
If you're doing this just to brag then I'm going to suggest you don't. That has a chance to backfire on you if she tells him, you did that and he doesn't like it. If she's constantly hanging around and trying to keep contact with your boyfriend though then I'd say go ahead because she should keep her nose out of your relationship.
It's called making yourself feel good.
It sounds like you just want to brag so I think you should reconsider. Its not nice to put things in people's faces like that. If she causes problems then maybe let her know but otherwise no.