No way to disconnect the OAuth2 integration app from Dropbox via an API call

To turn off the webhook notifications for a connected user, the user would need to disconnect the app from their account, which they can do via https://www.dropbox.com/account/connected_apps so you may want to ask them to do so. This is a manual process of course, so that's not ideal, and I've sent this along as a feature request for a programmatic alternative.

Unfortunately there isn't a good workaround to use instead directly from the app or developer's side. If the app is in development status and you want to disconnect all users, you can use the "Unlink all users" button on the app's page on the App Console. That would prevent any further webhook notifications as well as any/all further API calls for all connected users; there isn't a way to do this for just some particular user(s) though.