Assemblies have the right to regulate who speaks in your assembly or who doesn't. If you want to take that right away from them you deny them their right of assembly, free speech etc. You seem to have a definition of dissent thats seems to include the suppression of people you dissent with.  

But that isn't dissent. Dissent is speaking in your own assembly, demonstration and so on. In public spaces this is quite easy to solve: first come, first served. The counter demonstration has then to choose another space or time.

In private spaces the owner or operator decides.

Generally speaking you can't exclude a certain sort people - Blacks in your example - from a private space open to the general public. But you can exclude certain sorts of behavior.

whites only - not legal
shopping only - quite legal