Instead of turning the error logging off, have a look at those log files. The error may be due to some plugins or some bugs in elgg. If these are due to bugs in Elgg, you can report it to trac or post it here else you can report that to the plugin authors.

Yes, you're right Team Webgalli - I'll run through a log and try to extract whatever seems important.

Thanks.

(But I'd still like to be able to turn it on/off at will.)

Deleting a user entity consequently deletes any references to the user, including owned items, metadata and annotations. So, if you delete a banned user, there will be no record of him being previously banned, and he/she can reregister

Thanks Ismayil - that's clear now.