This will make sure anything asked follows with the question mark at the end of a question:

on $*:TEXT:/(?<=\x3F)/S:#: {
  msg $chan Original question: $strip($2-) And the answer is:
  var %8ball = $rand(1,11)
  if (%8ball == 1) { msg $chan No! }
  if (%8ball == 2) { msg $chan Yes! }
  if (%8ball == 3) { msg $chan Of course }
  if (%8ball == 4) { msg $chan Have you gone mad?! I think you have }
  if (%8ball == 5) { msg $chan How should I know? Its not like I'm magic }
  if (%8ball == 6) { msg $chan Heck no! }
  if (%8ball == 7) { msg $chan Maybe... yeah }
  if (%8ball == 8) { msg $chan Why should I tell you? }
  if (%8ball == 9) { msg $chan If I say yes will you be quiet? }
  if (%8ball == 10) { msg $chan Yes... that will be 30 dollars }
  if (%8ball == 11) { msg $chan How should I say this? Um... no }
}

Last but not least, you could have chosen the local variable instead of the global variable. This uses the same command that you've indicated.