Itay is on the right path, assuming you are using Sales Insights - if you aren't, but are using SFDC you can still have a connected program (linked to SFDC campaign) that the user can make them a member of, then have a listening campaign that does the enrollment. This makes it a definitive choice rather than something that can be an errant link hit.
I do also like Kim's comment on lead status or similar device - while qualifying you may select additional indicators that can be reacted to on the Marketo side and control the logic from their - so you don't put them back through the same campaign, or if they actually qualify higher for an alternate campaign.