You can set the "protected" flag on those accounts, preventing the users from modifying any aspect of their account (including display/notification settings). This may be a little bit too restrictive for your users though.

The fine level of control you're seeking is something that will take a lot of work to implement in Mantis. I imagine it'd be something requiring a full blown effort to make Mantis support a more advanced method of access control such as group DAC (http://en.wikipedia.org/wiki/Discretionary_access_control) or possibly even MAC or RBAC.