To create a Membership Level that is not accessible by the public, setup the Membership Level as normal but keep the 'Active' box unchecked.
This will allow you to still register Contacts to the Membership, but won't allow the general public to sign up to it.
To set up a Life Membership, create the Membership Level, but put a long expiry date (e.g. 99 years). This effectively makes it 'non-expiring'.
Don't forget to leave the "Active" box unchecked (so people can't join when they register).
Hint: If you also select "Auto-add Member to Group", you'll have a handy Group or Groups created when you add a member.