Any jamming attempts would have to be localized within a certain radius. Jamming an entire country's worth of area is probably untenable unless you are a really small country. Even then you could e.g. route communications across the border with a meshnet and then shoot them up to Starlink in a location that isn't being jammed.

This is the castle and invader problem; the castle builders must guarantee that there is no hole anywhere in the castle, and the invaders only have to find or make one hole to render the castle useless. Whereas you could cut an underwater cable and guarantee outages for at least a few days or weeks, an adversary only needs to move to an un-jammed area in order to resume communication with the greater world.

> What's preventing jamming a small, distant satellite signal with a big strong local transmitter?

Anti-radiation missiles which home on the strong local transmitter.

I'm not saying this flippantly. Obviously not all Starlink consumers have access to such a thing. The scenario I'm talking about here is when a country is trying to choke the military communications of an other one.

The big benefit of these LEO satellite constellations is that they have hundreds of satellites in them which makes anti-sat weaponry not cost effective. (Or at least that's the hope.)