1. There is a failsafe method of avoiding plunder. It's collecting your things on time.
2. If you are up against an experienced plunderer who makes a note of when your goods or FPs are ready for collection, alter your collection time. You don't even need to put your city shield up all the time. Just put it up for two days, alter your time to a more convenient time for you, and then you're set.
3. I find the practice of deleting FP-producing buildings particularly risible. One hoodie plundered by me saw her TF plundered for 5FP once in ten days. She deleted her whole city. Even less drastic forms of petulant deletion are foolish: let's say you delete your Masquerade Ball because I've plundered it three times in a week. Even then, you lose 8FP from your city over the week that you could otherwise collect, or 416 FP over the year. Unless you are being plundered literally every day, it's utterly unjustifiable.
4. If you reach, say, LMA or CA and the loss of one or two goods buildings a day is causing you significant angst, you're doing FOE wrong. By that time you should have a strong enough military that GE is straightforward until L3 or so, and you should have other methods of collecting goods like the CF, Tribal Squares, and recurring quests. I've been in CA 10 weeks and have between 2500 and 3000 of each good in my age - and I'm not even especially good at FOE.
5. Why do I take lower-age goods? Well, because they still have value. If a high-level guildie is going to sell me goods for a Kraken and is happy for unfair trades no matter what they are, I'd much rather trade 100 iron for the goods I need rather than 100 tar (I'm in CA). So these low-age goods I plunder either go into these unfair trades, or are traded and donated to my guild treasury for our IA campaign. It's not bullying, it's not sadistic, it's just strategy.
6. You're right that high attack boosts from the plunderer are hard to counter without really high defence boosts. So just plunder yourself until your gains outweigh your losses and get on with life.