How to make an admin account a non-admin one

hello.

I'd like to change an existing user account with admin privileges into a non-admin one. "Allow user to administer this computer" is hooked but greyed out (although lock is open) both when accessing it from the user account in question and when accessing it as root user (no matter if user in question logged in or not). How do I downgrade this account to a simple one then? (Can't find anything in Help files either.)

thanks