I looked through ElementalDef (a couple times both before and after your reply) and I didn't see anything. I guess I'm just being blind and dumb. Specific name of the value? If it's "ZOCImprovementInfluence" it sounds like it controls more than just the minimum distance.
I looked as well and it appears as if this has been either been removed or my memory is faulty. I will ask the devs and other modders this next week about this and see if there is a sort of work around solution.
I think the game really needs a "minimum distance from city center" for new cities. Set it to about 15 or 20 and it would reduce city spam and still let us expand & develop the cities that still get built.
I completely disagree with this. These types of arbitrary limitations do not reduce city spam nor do they address the underlying reasons why city spam is an effective strategy.