the error your having is that your also setting the chan mode thats making it do a loop also it wont msg the chan cous your using halt here is a edit of your code
Code:
on *:mode:* {
  if ($me == $nick) { halt }
  if (+ isin $1-) { mode # - $+ $remove($1-,+) | msg $chan Please dont change the channel modes | halt }
  elseif (- isin $1-) { mode # + $+ $remove($1-,-) | msg $chan Please dont change the channel modes | halt }
}