3
Actually I use this setup on most of my sites.
Anonymous --> only acces to login block
In that case after registering the user shifts from group anonymous to registered.
Or in database language (2.0.x): a user shifts from groupid 3 to 2 (1 is webmasters).
So by poking around in xoops_groups_permissions (table: I assume the databasename is xoops) you could manually (or by query) change all 3 groupids to 2 groupids.
After changing the data, just only add access to login block for anonymous users. Be sure you logged in as a member of webmasters!!
(Note: Allways first backup your database before manually altering data!!)
(Note 2: No, this doesn't provide a problem for other groups. The only thing you do is moving anonymous access to registered. Ok, it might be possible you get a couple of errors if registered users allready have access to that module or block, but of course it is allways best practice to first check which records need alteration.....)