Lemmie guess, this is CR (Conference Room) with +j on... short answer: it is impossible to stop people getting around this unless teh java client has heaps of stuff extra coded into it.
Long answer: Any proficient scripter can write a proxy in mirc script, and connect to it. From there, they can control entirely all of the output/input to mirc, and halt all sorts of /ctcp replys etc. There's no way to detect people who do this, if they do it well.