Depends on the satellites (for GEO ones it's very different), but for the particular case of Starlink, there are two aspects - first, as satellites are relatively low, each satellite can only possibly "see" a ~1000km radius so disconnecting certain large regions is trivial by having the satellite not work when it's above that area; but the main issue is that an important component to ensure their bandwidth is 'beamforming', focusing the antennas to cells which IIRC are something like 50km in size, so a Starlink satellite can be physically reachable for one city and not in the neighboring one, without the ground-based receiver being able to do anything about it.