How can I disable a light? (without adjusting its “dim”)
How can tell Houdini not to render this particular sop or object?

There's an active lights field on the parameters for your output (ie mantra render) by default it is set to * (everything) but you can change this to be whatever lights you want to use from your scene.

For future reference you can do this on a per object basis too, look at the light mask field on the shading tab for each object.

For a quick solution, do as Simon said. The exact syntax to exclude an object from a * (select all wildcard) is:
* ^fill_light
where the * means grab all lights in the scene and the ^ (carot symbol) followed by the name of a light object means “remove me from the list”.

The more robust way is to start using bundles to create categories of lights and manage the lists there.