JP is in no way liable for a discrimination suit. That mostly (if not only) applies to businesses and such when it comes to employees of that business, or government organizations for same and who the organization serves. This site, however, is not a business hiring employees from this site, or a gov. org..

(btw, even if he is hiring from here, he is still allowed to refuse those that he believes to be harmful, with reason, to his business)

This is (as said before) a discussion forum, and the owner can allow or deny access to whoever he wants just like he can do in his own home, or with his own belongings.

As for libel, I'm pretty sure you mean "liable", because I don't see how libel relates to that.

For proof of both being allowed in private settings such as this, look at, say, the KKK or other white-supremists.