Open system prefs and select iCloud.
Untick Keychain, it will ask for your Apple ID. Input that.
This should switch it off. If you want to reset it and then not enable it. Retick Keychain, it will ask for your Apple ID. Input it. It will then ask for your Security code. At this point, click reset Keychain, follow the prompts through, deleting from Mac or not as per your preference. Once you reach the point where it asks you to set up a new code, cancel out of this. Job done.
Keychain will remain off until you want to start it up again. Any details other than Credit Card or web passwords will remain in your Utilities Keychain, depending on whether your deleted them from your Mac or not. They will be listed under the header Local and not iCloud within the Keychain app in your Utilities folder.