More than once, members on my site have started checking boxes in their usersetings without knowing what they are, and ended up hiding their "User Login" and/or "Site Menu" which are both necessary blocks.

How can I disable that as a (checkbox) option in their user preferences?

tia

You could try removing the {boxes_block} variable from the preferences/displayprefs.thtml template file.

Better try that out first, as I'm not sure which defaults it would use when that block is missing - it might as well hide all blocks ...

bye, Dirk