It is "impossible" to disable the version reply. The people who have done this have ILLEGALLY modified the mIRC exe file, there is virtually no way to stop that. Code could be added to detect the change, but then at the same time they change the version reply, they could change the code that detects the change. There really is no defense.
But on another note, I wouldn't say anyone who doesn't want their version reply is "shady" I'd be very happy if mIRC let you hide the reply. The first rule of preventing exploits is never let other people know what software and what version of the software you use. Once they have that, the job of hacking/exploiting you becomes many many times easier.
