Unless they are posting a java script to automaticaly got to a url. There is nothing you can do. If they do use java to do so then you can turn off html but that is about all. One solution might be in your bad words censor to put in either www or http that might work.
PS just tried it myself. Here is the post in my own GB.
#@*%! and welcom to #@*%!.ravenprom.org and
#@*%!://ravenprom.org