Script Summary: Check off comments to delete, then blast them all at the same time. Use with Checkrange for extra fun.
Blogger Comment Moderation can be used to delete multiple comments at the same time. On a comment page (and while logged in to Blogger), just check off the comments that need to go, and click the button at the top or bottom of the comments section. No fuss, no muss.
If you need to delete a load of them, check out Jesse Ruderman's Check Range, which makes my user script twice as nice.