Raids are, due to the nature of raid movement, very difficult to stop or prevent. In fact, province disruption raids are essentially impossible to stop in most combat situations. Luckily, province disruption is not very painful. On the other hand, special drone and fighter raids can be very dangerous, particularly drone raids versus slow units and bases thanks to the ease at which drones can catch them (and the resulting doubling of drone factors). The most obvious defense is to pin out the raiders, but this tactic is typically difficult and under any circumstances uses an excessive number of ships (particularly in late General War with large raid pools), as most hexes can in theory be attacked from each of six adjacent hexes. So what can you do for raid defense?
Use the old adage: "The best defense is a good offense."

Raids must be launched from a valid Strategic Movement node (a Node that can connect to the capital), or using on-map assets. The range of on-map asset raids is limited. So destroy those opposing Strategic Movement nodes that threaten your positions and position your ships with Operational Moves to block Strategic Movement to those Nodes you have not destroyed or will not destroy. (Remember to block Strategic Movement before the destination node, as recent rulings allow Strategic Movement into some nodes that are adjacent to enemy ships.)

For on-map assets, cripple them, drive them off, or avoid their short raid range (pin forces in the relatively limited attack hexes available to on-map assets are sometimes viable). In the meantime you simply have to keep your FRDs, convoys, SAFs, and auxiliaries out of raid range.