Firing everyone in a store in the process of unionizing isn't "make employees not want the union", it's retaliating against those that do and instilling fear in the rest.
You might not like the methods (and in some cases, they are not legal), but I think that’s exactly what it does. Fear makes (some) people not want the union.
Firing pro-union employees also obviously directly reduces the number of employees who are pro-union.
Firing pro-union employees also obviously directly reduces the number of employees who are pro-union.