1. Edit the membership form part
  2. Mark the Show Current Membership(s) box
  3. Select the membership you don't want members to be able to join from Membership Category drop down menu
  4. Click the Remove button
  5. To keep users from renewing membership as well, unmark all of the checkboxes for Renew for the memberships.
  6. Click Save

Now all of the current memberships will display for the logged in user, but no new members will be able to join that membership.