If you block that instance that's doing bad stuff EARLY:

1. You can actually *prevent* abuse rather than just feel bad that it's a thing that happens and wonder why nothing could be done about it

2. You can influence the other instance's policies if they see quick and decisive backlash

3. You can prevent an instance from becoming "too big to block" in the minds of others

But you have to actually block them at the FIRST sign of trouble, not after the damage is done