Without farms all your planets are sitting ducks |
Shoot down troop transports before they reach your planets, then.
I've only ever been invaded once, and that's because it was a distant corner of the map, and I had no idea that the enemy I had just attacked had a troop ship nearby. I survived, but barely.
I don't agree with the idea of making planetary invasions harder. They're already barely worthwhile; it's usually more efficient to nuke their military ships and conquer them with influence. Especially if they have multiple planets in the same system.
However, I must say that the best part about being neutral is being able to use gas/mass drivers on invasions with few consequences.