I found another workaround in the Bubble documentation. This won’t be appropriate for all kinds of apps (for example, an online store where users can add items to a shopping cart before logging in), but there’s a setting that can be turned on: “Do not set cookies on new visitors by default.” This prevents temporary/anonymous users from being added to the Users data type.

My app doesn’t need to store any cookies until the user logs in or signs up, so I’m turning this setting on.