How to only keep passcodes on one device

Disable iCloud Keychain in the other devices -> Set up iCloud Keychain - Apple Support. You’ll be prompted if you want to keep local copies of the data, or delete it from the device you’re disabling iCloud Keychain.

Is it still one of your trusted devices? Does it show up in https://www.iCloud.com under your AppleID? If so, you can remote erase that device to remove everything on it.