Hmm...my apologies.. I based my earlier response on my knowledge of making hotkeys work for things like dialogs, without actually testing it. mIRC, by default, already has Alt key assignments for all of the letters in the word Ignore, so then I tried to find something that didn't come up with a default assignment.

Further testing has shown that this is incorrect, however, the following alias does work
Code:
F3 {  if !$chan {    /ignore $$1 1 | /closemsg $$1  } }


It's not exactly what you were looking for, but it's as close as I was able to find that worked.