I would think that blocking her number would be the best place to start.....if she does talk to you more than just texting.....then I would deal with it at that time.....of course, if you block her number she will probably come & talk to you about what's going on but then again, maybe she's not so bright & won't figure out that her number is blocked & that you aren't just ignoring her.

If she doesn't get the picture.....then just let her know simply & plain that you don't associate with people who talk bad about others behind their backs & she needs to go find other friends who will tolerate her behavior.

No need to get into a discussion or a debate....just the FACTS!!!!

Good luck