>Can we also make a hostile work environment to pro-union employees?

https://en.wikipedia.org/wiki/Union_busting

Historically, it would be hard to argue otherwise. It is lucrative to remove bargaining leverage from labor.

It already is hostile to pro-union workers, usually. This is part of the resentment that sets in. After months of surveillance and harassment, your pro-union coworkers will be pissed that you chose to side with the people who surveilled and harassed them.

What...sort of environment do you think caused them to unionize?

> Can we also make a hostile work environment to pro-union employees?

That's already the default in any non-union workplace, which is to say, the majority of workplaces.

Just move the office to a low tax country / state. Union people will refuse to move there.

The people who could create that hostile work environment refuse to cooperate with each other.