iCloud Keychain remembering old networks

I use iCloud Keychain on my iPhone, iPad and Mac. Recently, I've noticed my iPhone attempting to connect to wifi networks while wandering about town--I really only want my home and office networks to be automatic connections, simply because I've had a number of unreliable connections, particularly while walking downtown.

When I sit down on my Mac, I've noticed about 20-40 "old" networks I've attempted to have my iPhone/iPad/Mac all forget. Even if I remove them from OS X System Preferences : Network : Advanced, they re-appear after a day or so. I've gone so far as to Reset Network Settings on both my iOS devices. And I've reset my iCloud Keychain twice, in an attempt to prevent iCloud Keychain from syncing incorrect or stale data. Yet, every few days, I notice my iPhone has re-connected to an old network (commonly "xfinitywifi" or "ATT wifi" near a Starbucks).

What can I do next, to truly forget and remove these old networks?