Yep, there's no way to turn off Cross Planet Attacks, what you're seeing the setting for is making waves into a Cross-Planet thing (which ups the difficulty, not the other way around). CPAs are a pretty integral part of the game's general flow.
Part of the problem is that you're trying to use your homeworld as your choke point. This is bad bad bad, because there's stuff that can slip past force fields, and there's just no margin for error. It would be the equivalent of putting all the defenders right in front of the goal as soon as the other team has the ball in soccer -- like techsy says, you need defense in depth in some form to beat CPAs (and honestly, if you can't handle a CPA, then when you get to the AI homeworlds the spillover from those fights are likely to kill you as well, so you want to get the hang of defense in depth in general).
Make no mistake, CPAs are one of the most common times for folks to lose -- I think I lost most of my recent games during those, though definitely not all of them. What you really want is at least -- IMO -- three planets that the AI has to go through to get to your homeworld.
Three planets, all in a line, that you know they'll hit, that is. Then put turrets on primarily the outermost one and less so on the next two. You want enough on the inner planets so that any AI raid starships that are coming through will get trashed before they reach your home planet, but then you want all the rest of your ships and turrets and mines and such to be on that outer planet to deal with the CPA. Obviously you have a mobile fleet that is probably normally split up on defending other locations and attacking the AI, etc -- when a CPA is announced you have plenty of time to bring them all home to your last-stand location.
When the AI attacks your bottleneck stronghold planet, this is generally what I try to insure is happening to them:
1. As they come through the wormhole, some of them are flying through minefields, others are getting tractored, and the remainder are getting shot at by my giant fleet that is waiting there.
2. Anything that slips through, which will be a lot of stuff given the size of the incoming CPA, is getting shot at continuously by my ring of spider turrets that are slowly stranding their ships. I then ignore the stranded ships to focus on the ones that are still mobile, and come back to mop up later.
3. I have such a strong economy (I'm economy-focused in my RTS games) that I can have my entire fleet (starships and golems aside) die in that first encounter with the AI ships coming through the wormhole, and by the time the AI clears the wormhole and gets to the other side of my planet, I've completely replaced my fleet and thanks to the warp gates they've popped out and are waiting to deal with the AI at my command station on that outer planet.
4. My replacement fleet will fight them again, and will probably lose again, but will take out tons of their ships. At this point their forces are hopefully thirded, including all the stranded ships. They will then proceed to knock down my 1-2 force fields around the command station and then take out the command station. I don't really care.
5. By this point I have a third fleet that's being built and warping into either my second or third planet in the chain of planets leading to my homeworld. Depends on how well I'm doing up until this point. If it's going poorly then I go for the planet closer to my homeworld, being more cautious. If it's going well then I go for the planet closer to the AI, to head them off early and minimize my losses.
6. If it was going poorly already, then the AI will quickly swarm my second planet and take it. I'll build minefields and such to have them incur more losses, but it's incidental damage. I might throw some lightning warheads at them as they cluster around wormholes, and that can do some substantial damage. If things are really REALLY going poorly, then I'm prepared to nuke this specific planet and reap the AIP increase while at the same time stopping the CPA stone cold. Whatever was mark V or otherwise nuke-immune then gets through and my waiting fleet on the third planet takes care of them.
7. If it was going well, then it's just a fight to the finish on that second planet using conventional means. If I lose that, that's okay, as I can then just repeat on the third planet and still they didn't reach my home planet.
8. Sometimes a few ships get all the way to my home planet and so it's important to have turrets and some mobile ships there to deal with raid starships or whatever it was that slipped through (usually it's those slippery ships that lead to my loss, not the mass of ships that is the big distractor).
Unless the AIP has really run away from you and you have comparably low numbers of ships for what the AIP value is, you should be able to turn back most CPAs this way. You'll lose some planets, but that doesn't cost you anything permanent since you can have the colony ships pre-built and waiting. The trick is making sure that your starting planet can support this sort of strategy. When you're starting out, choosing a map and a map style that is conducive to this is a great thing to do when you're newer to the game or if you're a bit of a turtle like me.
The more open maps like grid or hubs or similar are really vastly more difficult and for a completely other playstyle that I don't play myself. I play grid occasionally, but I don't find it that fun and I'm not sure I've ever won a grid map game. I tend to like realistic, simple, tree, snake, X, concentric circles, and the new maze maps, and they tend to support my playstyle well.
Hope that helps!
