I just imported data that I should not have - is there a way to ""undo my last import?

if you didn't overwrite any critical data that you need - you can create a report that either filters on record ID# or date created to get all the new records that you imported, then use the more>delete option, then re-import.

If you over-wrote critical data in existing records that you can't re-import (i.e., you don't have a offline master copy), you've got a different issue