Just so that those who come after don't have to wonder why "unregister deleted dll" doesn't even give a Googlewhack, here's the answer. Download something called RegClean 4.1a. Run it. Hope for the best. You won't find it at microsoft.com any more; they've apparently stopped supporting it (possible answer why). You can still google it up pretty easily, however.

Update: Of course that didn't work. Get the name of the dll from your VB6 IDE (Alt-P-N), open regedit, select "My Computer" at the top (very important), hit ctrl-F, and start a-searchin'. I've been deleting the entire folder any time I found any key with the dll's name, and so far so good. Scary though.