What you're wanting to do, is called halting the default display.
Once a line appears in your mIRC window (channel or private, is irrelevant) you can't remove the line from your window.
For joining you want the ON JOIN event.
For parting you want the ON PART event.
For quitting (which is when a person leaves the network and all channels) you want the ON QUIT event.
To halt the default display you use the haltdef command.
Example code:
on ^*:join:#:{
if $istok(list of nicks you don't want to see the join information for space separated,$nick,32) {
haltdef
}
}