We have had similar situations here, and while we don't have a policy, we have approached the issue as a potential lawsuit against the district. As such, we contacted the district chairman and briefed him about the liabilities the district faced if it did not step in and stop the situation. We have an assistant attorney general assigned to the state agency, so we let her do the talking about the potential liability. Does your law have language that supervisors can sue and be sued? Ours does and using that helped motivate the district. Also, the district needs to do a formal investigation on the issue, complete with interviewing the person being harassed. The investigation should also include an interview with the supervisor to determine what are the issues motivating the behavior.
And, one last thing, does your state law have a provision for removing a district supervisor? If so, the board should seek outside counsel's guidance on how to move forward.