I just tried this on my test system and I could nuke a user OK.
So the trigger's on a table that the user you're trying to nuke has done something with.
It would help a lot if you could turn Verbose on for the main and subquery pools, restart AOLserver, try again, and post the relevant part of the log file so we can see exactly what "delete" is causing it to die. Once we know that, we'll know which table is being deleted from, can look at the triggers on that table, and fix the problem PDQ.
