1: Intercept them short of yoru planet.

2: Keep ships in orbit. This won't stop them if they are armed better than you, but it may slow them down enough to follow #1

3: Make peace, then bribe them to attack someone else. Or bribe lots of other people to attack them. Either way, they have more things to worry about than sending more spore ships at you.

4: Take it back just as quickly. A spored planet has essentially zero population for many turns, it's an instant win for you as well.

If you're going for the defense option, be sure to research far enough down the sensor branch so you see what enemy force is coming (in case they send attackers to wipe out your defense first)

Hi!

1) Turn off Korath.

2) Have a tiny ship with one weapon in orbit of each planet.

BR,  Iztok

Fortify border worlds with appropriate defenses. Are these spore ships armed? If not, a single fighter with a decent weapon can hold the ground.

What I do is put a cheap fighter in orbit around each planet near the border. Nearby, I keep a few battleships in patrol. If I see them, I immediately divert a battleship to intercept.

Custom made EW ships. The sensors on my ships have a range of 15, they're lightly armed, and they're fast.

Even so, I'll generally have to take back a few worlds, but the Korath will lose 60 spore ships doing it.

'2) Have a tiny ship with one weapon in orbit of each planet. '

I've come across armed spore ships in my games, so this tactic doesn't always work. One had an attack strength of 51.