Play with the On TABCOMP event, if you don't want to get sucked down a rabbit hole of rewriting mIRC's On INPUT behaviors.
You can examine the $editbox($active) and detect if the right-most character 'if ($right($editbox($active),1) == :)' equals a colon. Replace it with a goofy unicode chevron and /halt or /haltdef. Otherwise /return to take no special replacive action.
Edited by Raccoon (16/12/18 11:02 PM)
Edit Reason: i'm a maroon
Well. At least I won lunch.
Good philosophy, see good in bad, I like!