Go to "empires" - "strategies" -"firing" Set "do not fire on planets" true (green box during the game) for all strategies except "capture planet" Now your ships/fighters won't glass planet. If you want to capture planet, add Troop Transport with "capture planet", set "capture planet" to fleet and see how it works
If it does not go as you wish, play a little with strategies. Soner or later your fleets WILL capture planets.