Remove tags only within specific category

I have a bunch of posts within a specific category where I would like to remove a tag. For example, I had tagged posts within my “Twitter” and “Last.fm” categories with “self” – but I would like to remove the “self” tag from all of the posts within “Last.fm” only.

Is there an easy way to do this? It’s a lifestream, so there are dozens, if not hundreds of posts within each category. Manually editing each post would take ages.

I can navigate to phpmyadmin, but I’m not familiar with running queries, although I wouldn’t mind inserting a command if I was given one verbatim.

Thanks in advance,
J. J.
http://zepfanman.com/test3 if you want to see the lifestream