Ial

Turns IAL on or off. Note that this setting is not persistent across sessions and resets to on every time mIRC is run

/ial [on | off]

Note that you shouldn't turn the IAL off.

If you really want to turn it off, even across sessions, you need to execute the command every time you start mIRC. You could use the following script to do this:

on *:Start: { ial off }