The whole point of Lightmass is to pre-render lighting, and it does this largely by calculating light collisions and radiosity on the CPU.
It uses all of your CPU. That’s what it’s for. That’s the whole reason we have powerful, multi-core CPUs!
If you’re not OK with the CPU using all the cores for cooling purposes, then the problem is with your cooling.
I suggest underclocking the CPU in the BIOS to make it run cooler. This will achieve the throttling and perhaps lifetime improvement you seem to want.
Another option is to build a render farm of machines, perhaps running in the cloud, that you can start up when you need to build lighting. This will farm out rendering work to those machines, and the render swarm manager even lets you set how many threads are run on each of the rendering machines.