Answer
Set the mail server host in the Email Server settings screen to a dummy/non-routable address or localhost (which is actually the default value) — with no real mail server configured at that host, emailing fails silently across the entire instance, effectively disabling all outgoing email everywhere in that specific IFS environment. This is an instance-wide setting, so it affects every screen and feature that can trigger an email, which makes it a simple, effective safeguard for non-production environments specifically.