Using Moodle version 2011033002.04 release 2.02+ Build 20110427 Cannot unassign system roles. Page post backs no error nothing from debug and writes to log unassign role. But User remains in list. Happens under Manager and Course Creator.
Post Screenshots so it may give some ideas .,,,,
On this screen I would select User I wanted to move, then click Remove button circled in Red. Moodle thinks and then post back to screen. No errors, but user remains in the existing list.