This is a little /warn snippet that I am using right now, and it works with mIRC v6.03, because I've used -u instead of -ku. After 10 warnings, the script bans them for 50 minutes, and then kicks them after 3 seconds. Thanks to sailoreagle for telling me why the -k doesn't work.

; Alias Warn by UTUCS.com-Admin <admin@utucs.com>
;
; Put this script in your aliases. To use it, just
; type /warn <nick> <reason>. It will kick them,
; and after 50 minutes, it should reset the variable
; with the user's warnings in it.
;
; After 10 warnings it will kick them. If you shall
; have any questions, feel free to contact me using
; the comments system, or my e-mail address
; (admin@utucs.com).
;
; A thanks goes to sailoreagle for telling me why the
; -k didn't work in mIRC v6.03, then I updated it to
; -u and did /.timer. Works with probably any version
; of mIRC.
alias warn {
  inc -u3000 $+(%,$1,warnings)
  kick $chan $1 Warning $eval($+(%,$1,warnings),2) (Reason: $2- $+ )
  if ($($+(%,$1,warnings),2) > 10) {
    ban -u3000 $chan $1 3
    .timer 1 3 kick $chan $nick 10th warning -- please follow rules!
  }
}