Added option to ban users for updating only

Reason: I want to disable enqueueing users like spacecowboy, because MAL
returns 500 Internal Server Error in some documents for such users. This
effectively renders full profile updating impossible; right now the user stays
in the queue forever, being unable to update and slowing things down for other
people. I don't want to implement partial profile updating since it would add
too much unnecessary complications. Once MAL resolves problems with these
documents, profile updates for problematic users will be enabled again.