If you want to shut off the ability for people to sign up for a membership level past a certain point, you could set the status of a membership level to inactive which would disallow signups from that point forward and would not impact those customers who were already on the membership level. 

If you want each customer who signs up for the membership level to expire at a certain point (i.e. after 6 months or 12 months) then you'll just create an expiring membership level. To learn more about this see the Expiry Settings section in this article: Create a Membership Level