[Raapys]

The relevant strategy options lets you decide which targets to attack, in which order to attack the targets, and the distance at which to engage the prioritized target.

So you can have a particular design( or task force ) attack space based ships and units while ignoring planets entirely. However, there's no way to actually make them stay away from the planet itself when pursuing other targets, so they could be fired upon by the planet when chasing their objective.