Is there a plugin that is able to filter profanity in comments, or at least to force user to break up forbidden words?

i don't know of one for this specifically, but you can have certain words replaced with other words (that you define) with the macro plugin or the quicklink plugin.

This plugin was just released, it works with the WebPurify Profanity Filter web service.

http://plugins.movabletype.org/webpurify-profanity-filter/

Another plugin just released, too: Mouthwash. Disclaimer: I wrote this.

Mouthwash is a great plugin. Good name too!

Thanks! A rating or review on the Mouthwash Plugin Directory page would be appreciated, if you were inclined.