If you have broad support from the employee base, “they” can’t block a union certification election. If you’re having trouble forming a union, you’re probably struggling at the “get employees to want your union” step in the process.
You should let Amazon, Starbucks, Walmart and similar know this. Their union busting tactics are widely documented (including shutting down locations starting to form a union).
They do know this. Many of their tactics are specifically directed at “make the employees not want the union”. Some of those tactics are under-handed, even despicable, but it’s safe to say that they know this and act in accordance.
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.