The subject of people disconnecting their cities coinciding with getting plundered comes up fairly often throughout the forum. A few even insist (wrongly, of course) that it's a wonderful strategy to fix being plundered. Some will take what they learn from others here & implement ways of making their city more successful. Others won't or can't (for whatever their reasons).
Furthermore, I feel that plunderers aren't under any obligation to help others find ways to be better in any aspect of their game. You don't have to feel bad about someone disconnecting, either. Feel whatever you want, and share your experiences. I don't care.
Why am I here in your thread?
The vast majority of folks who are disconnecting have gotten to a point in their game where they feel frustrated & firmly stuck (or know they're very very close to being so). They've already stopped progressing adequately in their eyes. No matter what they do, they're unable to figure out any correct strategies to again begin having what they consider a successful city. Their situation is entirely of their own making (often due to limited understanding of game fundementals, etc). Disconnecting is simply yet another of their numerous failed methods/attempts at trying to find ways to be successful again.
Putting it bluntly, disconnecting their city is merely one of many symptoms of their own stagnant progress based on that individual's own self-standards. You, as a plunderer, are doing absolutely nothing to create or contribute to their own failing decisions/actions. They're clever enough to be accomplishing that on their very own.
Yet, you really want to take credit for something you have not actually contributed to in any way: "... Personally I think by forcing a player to disconnect his city is one of the two big victories as plunder."
By your own admission, the pinnacle of your own personal accomplishments in plundering - what you strive for most of all & collect "trophies" of along the way - is for having, in reality, done nothing. What'd you do exactly... make them feel more stuck than firmly stuck?
I find your primary measure of plundering accomplishment exceedingly more pointless then the actions of those folks disconnecting their cities. Very amusing.