Employees who are forced to work under conditions that leave them little choice but to quit can still sue, alleging they were constructively discharged.
You can prevent those suits by transferring the employee who says he is being harassed to another equivalent job. Removing the employee from the alleged hostile environment removes the cause for quitting.
Recent case: Markeith, who is black, complained that he was being harassed and discriminated against. He was then transferred to another position. He quit shortly after and sued, alleging constructive discharge.
The court dismissed that part of his lawsuit, reasoning that the transfer removed him from the allegedly hostile environment and therefore also removed his need to quit. (Thomas v. Hennepin Healthcare, No. 14-1842, DC MN, 2015)