They shouldn't be "forced" to continue associating with someone; they should not change their position on whether or not they should associate with said person based on this situation.
That sounds abstract, but such concepts already exist. If you have a restaurant, you are allowed to refuse to serve someone who happens to be a member of a race R, but you are not allowed to refuse someone _because_ they are a member of race R.
Maybe people are just very fickle these days, but last I checked: "someone posting on your server" is not association. Site owners put in that one article precisely so that cannot be the case.
But this does break the CC license by unattributing content but not deleting it, so that's bad.
I don't know if people are different these days, but we certainly have taken the concept "all relationships are voluntary" too far. If we had a society where people associated with each other only when there was personal gain to be made, that would not be a very nice society.
That sounds abstract, but such concepts already exist. If you have a restaurant, you are allowed to refuse to serve someone who happens to be a member of a race R, but you are not allowed to refuse someone _because_ they are a member of race R.