4
I assume you're talking about access rights, and not the rank/title that's displayed under a user's name. The latter has nothing to do with access.
The table xoops_groups_users_link controls users' group memberships. To remove a user from the webmasters' group, delete the row(s) with uid = (the user's ID) and groupid = 1.
There's also the mysteriously undocumented xoops_users.level column. You should only have to change that if you're changing the access for user #1. The values (I think) are 5 (admin), 1 (registered user) and 0 (inactivated user).
