Sometimes I forget just how many tools I've added onto BottomFeeder. Have a look at this post from this morning, for instance - where I went through a script I used to find feeds that had not been updated recently. I mentioned that after doing a few manually, I went ahead and wrote a script - to which Troy responded that I ought to have support in the tool itself.
Oddly enough, I did have such support (although there was a bug that could bite you on it, which I've just patched). Have a look at the System>>Feed Maintenance menu pick - it brings up a tool that looks like this:
It's a pretty simple tool - select the cutoff date, hit the button, and you get a list of feeds that haven't updated since the cutoff (by default, 60 days back, once you get the update). Select the feeds you want to delete, and then just remove them - boom, all gone. So Troy - the functionality is already there, I had just forgotten about it :)