What happens when the frontal lobes of the brain are overactive and a person has to much inhibition? What kind of mental disorders does this cause?

The idea that mental functions are localized in a specific region of the brain is false for higher functions. The highest hierarchical controls are provided by extensive networks across the entire brain.

There is also no pathological hyperactivity of these networks, except in the case of epileptic seizures. Let's talk about hyper connectivity of the control networks, which involves them in most mental matters.

This hyper connectivity does not correspond to an inhibition. The control exerted in particular by the prefrontal regions is an evaluation and not an inhibition. Inhibition comes from the conflict between contradictory mental representations, which is sterile precisely because of the lack of evaluation. Most inhibitions come from poorly managed emotions and impulses, not from excessive control by consciousness. What is prohibited consciously is not an inhibition. The reasons are understood. To feel inhibited in one's desires is not to have appropriated the assessments made by one's own self-consciousness.

 Reference: Jean-Pierre Legros