We have dealt with something similar...we've received lists from tradeshows that we are only allowed to use once, but if they respond and interact with us we're allowed to keep them in our database. To solve this problem, I turned off my Sync To Salesforce smart campaign, imported the list, and then turned it back on. When the Sync to Salesforce smart campaign is off, you can import a list and it will not be added to your database, the list will just live remotely in Marketo. When you turn it back on, your sync will begin running normally again. I hope that's helpful.