mIRC has a built in Yes No dialog, the only problem is that you can't use it from events, timers is a easy way to get around it.

on *:INVITE:*:.timerinv 1 0 inv $nick #
alias -l inv { var %x $input($1 invites you to join $2,y) | if (%x) { join $2 } | else { notice $1 No Thank You. } }

(thanks gutiarmasterx7 for code box) :D