I've had a different experience. I accidentally clicked on a different world but realized it immediately when it asked me to name my city (or something that occurs before you actaully go to the city - I don't remember what it was). In any case, I backed out of the process then but when I went back I found I had officially joined that server, the assumption being that I had started a city there. I hadn't, having backed out before that step. I wrote them asking them to remove me from the server and suggested a code change so that you don't join a server until you actually start the city (whatever the step is you take after clicking on the world), and while I don't know for sure, they may have changed the code, and after a couple weeks that world was no longer active.
When I say they may have changed the code, I accidentally confirmed this when a few months later I, again, clicked on the wrong world and, again, "joined" the server but didn't start a new city. Within a couple days my "new" server went back to "inactive." I've not repeated the mistake but expect the same thing to happen in the future should I have failed to learn my lesson and click on the wrong world again.
So, maybe you "joined" the server before the (assumed) code change. I can't say why they haven't removed you, but if you actually started a city there maybe the code doesn't allow you to go "inactive" once that happens (which makes sense if they can only remove a city -- once started -- by removing the whole account, including cities on other worlds?)
Just my experience.
AJ