Whether that should be the case is certainly subject to debate, but so far as I know the courts have always upheld the right of the owner of a "virtual space" or their assigned proxy to restrict both content and individual participation. In this sense, these virtual spaces are identical to privately-owned physical spaces.

Even when those spaces are open to public use, so long as that use is entirely voluntary, it can be made contingent to accepting specified restrictions. Use of the space is generally made to constitute de facto acceptance of those terms. Without looking it up, I seem to recall that Boardhost's ToS functions that way.