troubleshooting problem with [img] code in signatures

[AtoZ]

I have the global setting for img code in signatures set to OFF. However, when a user puts image code in their signature, it still gets through. I can't figure out where the problem is occuring.

I tried posting this at vbulletin.com but I couldn't any leads other than "uninstall all modifications and turn off plugins then it will work." First of all, I don't have a TON of modifications, I only have a few custom templates, and I am not even sure where all my hacks are since I've had vbulletin for so long. Of course I am not going to start over from scratch and break a million things to get one thing working.

What I was asking there, and hope someone will answer here, is where can I look to find the problem that is causing this? I'm not asking for someone to solve the problem, but give me some information about how vbulletin controls this setting so I can investigate and find out why this is happening.

TIA for any ideas.