Trim syndicated RSS posts

I’m looking for a script or plugin that allows me to trim posts of RSS feeds from other sites before they are presented on my own site. More specifically a post will start out with some unwanted content that I wish to remove. The wanted content is preceded by a string of text that looks like this:

KEYWORD DDMMYY

A plugin that handles both syndication and trimming would of course be nice, but since the syndication part is easily handled by a number of plugins it would also be an ok solution to trim the posts after that “arrive” to my site.

Thanks in advance ??