Hey Toni,
I would create an engagement Nurture specifically for those Leads. You can be straightforward and ask them to subscribe to your mailings. If they Unsubscribe, I would DQ those, pull them from SFDC, and put them in a wait step for 6 months. Then at the end of the 6 months, have them deleted. I would delete those who show no action after a pre-determined time period, whether it be 2 days, a week, 2 weeks, etc. For those who say that they would like to continue receiving your communications, keep'em in! You may have uncovered another target market.