Troublemakers opening bus door / luggage door to stop bus from moving

I haven't come across this with a while, but the other day I had loaded the luggage and was about to drive off. You've to close it from outside in the coaches we drive. Some teenagers came up and opened the luggage door without me realising it, and went back to lean on a shop windowsill. They obviously did it before the main door was shut. I got out and didn't look at them (so as not to encourage it) and closed it again. I was thinking I hope they don't get to it again before I close the main door, and thankfully they were too late. I assume they left it so late because that would actually be the best way to annoy someone... to wait until they're all settled in their seat before doing it again! I'm lucky they didn't know they could still have easily opened the main door from outside, and then did it again.

I remember in the Volvo/Wright double-deckers I used to drive, if you held down the green 'close' button from inside that would stop teenagers from opening the door using the outside button. Wish we were told that when we started during our training! I don't think they're any trick for doing this in Volvo/Plaxtons. What would I have done here if they just kept opening it? Could I have explained the situation to the passengers and seen if anyone was willing to guard the door and then meet him 100 yards down the street? Or else waited 20 minutes for Police to show up, just to realise that they'd had enough fun after 15 minutes?!

If I'd have given them a look as if to say "I know you did that you little scut" then they'd be quick to make me realise that they have the upper hand. Is there any tricks to get out of this situation? I know one bus type I have has a button to lock the main door from outside once closed. I don't know if Tourismos have this feature too, or not!?