Hi! At the moment, the only way to Blacklist someone is by either having him a friend at first and blacklisting him or clicking his name when he appears in World Chat and blacklisting him then. I do highly recommend reporting the individual with screenshots to misbehavior thread so that this issue can be investigated and proper action taken to resolve it. Screenshots should include Server ID and Time Stamp, which can be seen on the top left hand corner.

You can find the misbehavior thread here.

Please know that I am here if you have any questions =] I am available in private message.

~Jib