Apple does not list a way to guarantee removal from iCloud. Deactivating does not work, they'll just sync back from iCloud when it's activated again. You can activate the sync and then delete the passwords from your device one by one, as that syncs to iCloud it should remove the passwords from iCloud as well. That is the only way I know to come to a reasonable conclusion that the passwords are really gone. But then they are gone from your iPhone as well of course.
In any case, passwords are end to end encrypted in iCloud so Apple and third parties (through Apple) cannot access them:
https://support.apple.com/en-us/102651
I make use of the iCloud keychain sync because it's the only solution out there that reliably syncs across devices without any third party apps and without Apple being able to read the passwords from their servers. Unless you are in China where Apple cannot be trusted in terms of privacy I would keep using the iCloud keychain.