Ok,I had the same done to me by the slapper my ex ran off with (she actually done it in real life aswell though and I'm pretty sure there was something else she done). I've never met her either.
Anyway,first thing to do is contact the moderators of the forums to get it pulled.
If she is posting anything that makes you identifiable you can also contact a solicitor,but it's very expensive. The threat alone of that should make her stop. Be sure to tell the moderators of the forums that you will be instructing a solicitor if it is not removed,even if you won't.
You can also contact the police,whether they do anything is a different matter. If she is harrassing,stalking or inciting anything towards you they may.
With one forum, though, that aforementioned slapper posted about me on (I had never heard of the forum but a member from there actually contacted me to tell me was going on!) I actually signed up,posted on one of the threads she had done about me and told everyone who I was,what she was doing and what rubbish it was,they could tell I was being honest and her being a nutjob 
and she got lambasted and after, yet another, 'I'm sorry I lied,I didn't mean to' ( used every time she was caught,only to go and do the same elsewhere or under a different name...) the threads were closed by the admin who wanted to protect themselves due to her defamation and she was pushed off the site!