I abandoned a city in Khelonar; it was just too much to keep up with three cities (two on a different id).
All you have to do is leave the fellowship in those cities and stop playing. Simple. If you want to be nice, you could put a message in your city name saying "Not Playing" or "RETIRED" ... it will help your neighbors to know that.
I would suggest you leave the worlds viable. I played Darielle for 2 years before I got nostalgic for a city that my son created for me years before I started Darielle. I never had time to play it, and he kept it up for me for a couple months and then abandoned it. But about 6 months or so back I ran across the name on Elvenstats and I thought, "Aw, I'd like to see how that little city is doing." Since then, I've started building it back up after a 4 year abandonment. Sure, it was in the boondocks, but it didn't take long for it to be moved to a good neighborhood. I'm so glad I didn't disband it. It reminds me of Tom.
You might have other reasons for wanting to play it someday. You never know. I'd just stop playing if I were you (leaving the fs first, of course.)