[GameWizard]

Well the only way is to manually edit the newthread/editthread templates and add conditionals around the prefix drop down menu, which then will prevent those who you dont want utilizing the prefixes.

However the downside is that it will apply globally to all the prefixes, so you cannot specify which prefixes you want to appear, and which you dont. You can however limit this to only appearing in a paticular forum by creating a child theme and apply it to the forum of choice and apply the necessary changes to the templates within.

It's rather simple really, just find the conditionals for usergroups and make the necessary template changes.