If I'm not mistaken, you can set a config:

concrete.log.emails

This value should be set to false. You can do that using the CLI (https://documentation.concrete5.org/developers/appendix/cli-commands)

concrete/bin/concrete5 c5:config set concrete.log.emails false

Otherwise into /application/config/concrete.php. Read more about that here:

https://documentation.concrete5.org/developers/configuration-and-key...

I have tried doing this but I still get a saved report in the reports area.
Perhaps this configuration setting is just to stop the system logging emails that are sent?
Thank you for your help so far.

There is no way to do this without overriding the block's controller. The report is saved in all cases, there is no setting to stop it from being saved.

I also looked at the legacy form in Concrete5 v8 because it dispatches an on_form_submission event so I thought by using that event it would be possible to delete the report after submission. But it's useless for this purpose as it doesn't include the newly added answer set ID.

Currently not possible. I created an issue for it a few days ago:https://github.com/concrete5/concrete5/issues/6661...