this needs to be stopped, its the 3rd time its been turned off outside, ive an electric fan oven so only gas is bolier. you get sick of complaining though thats the problem, nothing done 
We had a similar prob, when i worked on a business park in Bristol......we had v. expensive main frames.....on test....we used to manufacture them....they needed a lot of cooling in the computer rooms....so the computer rooms had 5 or 6 air conditioners....running 24/7....the compressors/condensors were outside.....with the mains isolators next to them.....every now and then the local brats would come along during the night and turn the isolators off.....result was.....air temp in the computer rooms....rose rapidly....and basically the main frames cooked themselves....costing 10's of £1000 each time

The isolators had holes in them that aligned when turned on/off.....that could allow a padlock to fitted to stop being turned off/on....but health and safety wouldnt allow them to be padlocked in the on position.....but did allow a thin cable tie to fitted loosely.....so they couldnt be turned off.....but gave enough slack.....that an adult if they tried hard enough could break the cable ties off.
Could something similar be done to your gas valves?? :-/