[Blueentity]

As previously mentioned, if you set a ship's (not fleet, but ship) strategy to "Don't get hurt", it will "run for the corners" instead of approaching enemy ships, planets or units. Certain Design Types have this as a default, so unless you have modified that strategy, an uncloaked mine sweeper or a colony ship will blockade a planet quite nicely, until there are some defending drones, fighters or ships.

Even on “Don’t get hurt”, your ships with guns will defend themselves if an enemy ship or unit moves within its weapon range (Try it out in the simulator to see) if attacked, but will still run to the corners and leave the blockaded planet alone. The obvious disadvantage would be if the enemy defenders have longer ranged weapons than your ship (Missiles anyone?).