This is only an idea and might not actually be a beneficial game mechanic.
It should be possible to order starship fleets to blockade a planet. A blockade would cut off trade routes and prevent the establishment of new trade routes to the planet. Blockading fleets should auto-attack any fleet that tries to transfer down to the planet or transfer resources/troops up from it.
It should also be possible to blockade Mesophon planets to prevent them from being used to trade resources or buy ships. You should not be able to trade with any Mesophon world if you are actively blockading them.
Blockading planets outside your own territory should be considered a war act, negating social order penalties for the player you're attacking if they counterattack your worlds.
However, maybe blockading a planet whose 50 ly sensor radius is totally enclosed by the sensor radii of your own planets (that is, inside your "territory") should not be considered a war act. This would give smaller players an interesting way to harass larger players and maintain control of territory without permitting disproportionate retaliation from the stronger player. It would also make it harder for empires to set up sector capitals near or inside the territory of other players in advance of an attack.
If cutting trade routes and causing unrest is too strong, alternative effects of blockades could be:
- cancel or decrease the control radius of a sector capital
- prevent jumpbeacons from operating
- prevent citadels from firing jumpmissiles
The space forces rating required for a fleet to successfully blockade could be some function of planet TL and/or designation and/or defenses and/or # of trade routes, and there could be a non-unit-producing defense structure ("space traffic control") that raise the forces number required.
Jumpships probably shouldn't get this ability; they are so fast that it would be easy to screw with other empires while they're AFK.