You import people to a static list, send out the email, and later update the Marketing Suspended of all the people in list to True. With the Marketing Suspended field set to True, these people could still receive Operational emails sent from Marketo, if you don't want that, you can update Blocklisted field to True instead (although make sure you know the consequences of this, e.g., if anyone in the list signs up for a webinar, they'd not receive the operational registration confirmation email).
Alternatively, if you have a suppression smart list/segmentation, that all your email campaigns reference, then you can add a filter there so these people are in the suppression list. Lastly, between the time you're setting this up and updating their Marketing Suspended/Blocklisted to True post the email send, make sure to suppress these people from receiving other emails by including a Member of List not in "<List Name>" filter.