If an employee made it passed their 90 day probation period but things started going down hill after that, can u legally restart their probation over?

You don't provide any background information regarding whether this is public-sector or private employment, or whether there is a collective bargaining agreement or employment contract. In the absence of an agreement to the contrary, employment in the United States is on an "at will" basis. That means that the employer may terminate or unilaterally modify the employment relationship at any time for any reason.