There isn't a way to undo an import but there might be a way to remove the contacts depending on the situation. How were they uploaded--via file? If so, upload the file again into a brand new list. Click into that list, select all the contacts and click "Manage Lists" > "Remove From Lists" and check the lists you uploaded the contacts to by accident.
If you didn't upload via file it would be bit trickier but you could export all of your contacts (including the Advanced Info), open the file and sort by "added date" to the date you did the accidental upload. These are the contacts you will want to remove. If its a lot of contacts you can always add them back to the account in a new list as mentioned above.
Hannah M. Community and Social Media Support
If you find my post helpful, and it answers your question, please mark it as an Accepted Solution
You can post a new message in the Community or find us on Twitter Mon-Fri 8am-8pm ET, Sat 10am-8pm ET, & Sun 10am-6pm ET. We've got real people waiting to help you out. Click below to start a conversation!