How do I blockade?.

[Randallw]

Twice now I have tried to start a blockade and both times it has failed. The first time I told the fleet not to get hurt, that way they would stay at the edge of combat and not attack the planet only destroying ships etc that came after them. They ended glassing the planet because there were no defenses to threaten them. Next I told them not to attack planets but still they did it. How can I tell them just to stay away from the planet while blockading it?.