Are you deleting the user account on WordPress, or is the user’s membership just being cancelled on MemberPress? If the account is being cancelled on Memberpress, there might be something in this topic that can help you: How to prevent some WP users from being able to login to Discourse.
After preventing the user from logging in, you might want to go to their preferences page on Discourse and change their email preferences so that they no longer receive emails from Discourse. Another approach would be to suspend them on Discourse. This could be done through the Discourse API, but it’s not something that the plugin handles at the moment.