Mass delete empty categories

I’m using FeedWordPress on a pretty massive “planet” web site, aggregating around 50 blogs. FWP auto-creates categories to match all incoming post tags. Over time we’ve amassed around 6000 categories, which really weighs down the the WP interface. Now we’ve started removing all posts older than a month (no need to keep them around forever), which leaves me with thousands of unused categories. I’m looking for a plugin or method to mass-delete unused categories and nothing came immediately to the surface. Can anyone recommend a plugin or technique for this?

Thanks.