We just encountered this issue. In a perfect world, you'd have workspaces that would allow you to keep them separate from the rest of your database until they clicked and moved over. Do you have workspaces?
If not, the workaround simply is that you do import your list and do the send (make sure that this is in a program). Be sure to use program steps (perhaps using engaged or clicked for the leads you acquire and mark that step as success and perhaps use 'sent' as well for everyone to attribute them to the program; this will allow you to run a program success report later). Following the send, pull a list of people who did NOT click on the link (either through did not click or [if you used program steps] those who's status isn't engaged). Then run a smart campaign that deletes them BOTH from Marketo and your CRM. Let me know if that doesn't make sense.
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