This exploit is based on two facts:

1. The heat capacity of reactor increases with the size
2. The heat of a reactor is stored in the controller block

So generally, if you can build a reactor which is large enough to not melt a single piece of fuel, you can always break & replace the controller block to reset the heat. And this can be done automatically.
I'd suggest adding a NBT tag on the controller about its temperature on broken ( and prevent any method of stripping the NBT, like crafting), or there might be better methods.

This will be sorted in the future with the solid-fuel fission overhaul, when I move to using the multiblock API as I do for the MSRs ;)

Make it so that if you remove the controller from a heated reactor, it blows up on your face! Now I want to see those pesky "players" exploiting nuclear power =P

The overhauled fission reactors are in a semi-working state now, and heat can't just be deleted in this way anymore ;)