It's difficult to know if this problem could be foreseen, but a simple message in advance to the effect that "You may see some fuel coming out of the wing vents, this is normal and not hazardous" and no problem.
I do not think that the pilots have any direct indication that fuel is poring out the vents. They might notice that the amount of fuel is decreasing as a secondary effect. So notifying the passengers early might be tricky.
However I do agree with JW411 that the situation might not have been aggravated this much if the captain or the fo had come into the cabin and explained the situation calmly. But then again, it might have been to busy on the flight deck to take the time to walk to the back.
Beyond the call of duty for Flyglobespan to return the passengers, I would have thought. If they choose to diregard the information and reassurance of the Captain, in the process delaying all the other pax, then they should have to deal with the inconvenience of getting home themselves.
My opinion: When I make an educated assessment of the situation and decide that it is safe to operate the flight I think it is more than reasonable to let passengers, who decide to leave my aircraft, pay for their onward journey. Not to mention the cost incurred with the delays of off-loading them.
C-T