I don't think you can kick/ban them. One time while I was hosting someone renamed me and changed my character, team and fur color >(. Then some suspicious character just suddenly happened to join my server (which was passworded btw) named "Whatever". I was suspicious that this was the haxor and he/she would do something evil, so I typed in "kick 2" and kicked them. then my jj2 suddenly crashed right when I typed kick 2 and pressed enter. Coincidence? I think not.

And you can't actually SEE who is doing it. One time in Scizor's server there was someone kept joining and leaving under different names such as "FireSword DM, Trexian (BN's online name) and some other names. They even joined and left under scizor's name (who was hosting the server) and under my name (They matched the color and everything perfectly). And each time they joined and left either someone would be renamed or their character changed etc. Plus they did it so fast that that it was impossible to ban them/see their IP adress.