Nice advertisement..

www.mircscripts.org has plenty of addons for mIRC, but what you want is simple, just put this in your remotes:
Code:
on *:OPEN:?:msg $nick Please do not PM me unless it is URGENT.

And it will message "Please do not PM me unless it is URGENT." to everyone who queries you unless you already have a query window to them open.

Edit: Just because noone has responded within 7 minutes doesn't mean noone ever will, and asking people to respond after that time period won't make them respond any faster.