Stop new members from using sigs

In vBulletin you can set the primary usergroup so it cannot use sigs, to stop stupid spammers posting silly posts with porn links.

My secondary site has this problem but runs MyBB and I can't find an option to disable sigs.

The site is mainly used as an arcade so it wouldn't matter if sigs were completely disabled for everybody.

Is there any way to do this, in some form?

 

You can use this [Release for mybb1.4*] Minimum number of posts to create signatures