Limited use of attendees list from an event

We received a list of registrants from an event we're participating in next week - and the list is quite large.  We have permission to use their email address just once.  Have any of you dealt with a use-case like this before where you still used Marketo to communicate with them a single instance?  If so, what did you do after you used the list?  After determining which leads are net-new (and not already contained within Marketo), we would obviously want to delete the list so that we don't violate the usage rights.  And then there's the inability to report properly (or even obtain new known leads for those that respond to our offer).  Just looking to see how others may have dealt with this in the past.