Prevent certain users from sending IM?

I’ve been using OpenFire 3.6.4 for some time, but only selected users get to use IM (via Spark). Active Directory integration is working great. I would like to use OpenFire at my organization for presence notification and broadcast announcements company-wide. Management is afraid that employees will waste time IM’ing each other. Is there a way to set certain users or groups such that they can receive messages & broadcasts, have presence notification, but cannot send IM’s? should help.