Currently, we do not offer the ability to undo an import. However, there are two ways to locate recently imported records and removed them from your account.
Deleting Records By Tag
If you applied a tag upon import, you can locate the tag by going to your "Contacts tab" and selecting the tag to the left.
Your list of contacts will display and you can select everyone in this list then click the trashcan icon to delete them.
Full details for contact deletion here: Bulk Deleting Contacts
Deleting Records By Creation Date
Another way to delete recently imported contacts would be to perform a creation date based search using our Segmentation feature.
Start by going to your "Contacts tab" then clicking in the search bar.
Select "Add Filter" then choose "All Contact Filters" >> "Systems" and "Creation Date." Choose your date range and Nimble will display a list of contacts imported within that time.