It's probably a script you have loaded. Type /!remote off and see if it stops it. If it does then it's your script, either remove it or try this:
Type alt+r and search for "who #" or "who $chan" (or perhaps just "who" on its own) and see if you find anything, you can then put a ; at the start of the line to stop mIRC performing the /who. This may cause problems, depending on the script you have loaded.
Type /!remote on to turn your scripts back on.