Document the performance issues in writing, create an improvement plan for the employee that they sign, and if the employee cannot correct the issues, you fire

Document the performance issues in writing, create an improvement plan for the employee that they sign, and if the employee cannot correct the issues, you fire them. You need to do the same thing with all of the employees involved in the hiring process, or get them out of the hiring process if that isn't their primary role and they are otherwise competent.