An inlet like what is posted in the picture is not a problem at all . The prankster would have to have a pressurized hose to fill the tank.
Honestly that would be too much work for the average punk kid.
A gravity feed inlet would be a bit easier to tamper with. You could possibly drill the cap and filler neck and insert a pin or pad lock to secure cap.
Personally I have a lot of other things to worry about. I see no need to secure the inlet. However if you are worried it can be done.
Worst case you could add a hatch and secure connection behind hatch. The whole idea of it is a solution for a problem that does not exist.