Exactly what I was thinking. Remove the feature in one SQL command for all.
We tried asking the official Invision support but they did not help in this question... plus the version of this software is old, version 3.x, we would need first to upgrade to 4.x
Therefore, the final solution would first be to work on upgrading to 4.x, please test the software via the proxy server I gave bizman.
Then go live on 4.x.
Then test SQL commands, or hire a programmer to teach us the command to do so, or teach us how to custom remove the email notifications for topic subscriptions feature in PHP.
Which gives me an idea to ask a php programmer to look into just disabling the sending of email notifications in this existing 3.x program.