Except it already is exactly that way, anyone can block anyone else they want. It turns out blocking people does nothing when you're being harassed by big groups of people, the damage was already done before you got the chance to hit block. And also it doesn't stop people from trash talking you behind your back and furthering the damage.

And there is the other dimension where you actually want to remove the ability to block from some users, like public businesses and brands probably shouldn't be able to block complaints from customers and shareholders, politicians shouldn't be able to block their constituents, etc.