There isn't any feature to report a personal text on the forum to be a threat and I don't think it should be reportable as it is a users personal advertising space.
I don't even think it is that serious of an threat though, rather just an move out of spite.
So you think people should be allowed to post content that is not allowed in a post or a pm to be displayed in their personal text and signature space? Also, when it comes to advocating to "K*ll" someone, don't we need to draw a line somewhere? I really think at least a temporary ban is in order here.
I went ahead and sent a PM to theymos on this matter, to see if he can weigh in.
Edit: Banned for 30 days. Locking the thread.