d9780 The advice you received from MySudo is correct. Force quitting an app doesn't stop it from being started again. If you want to ensure that the app can't access your IP address when you disconnect from VPN, you should temporarily disable the network permission for MySudo, or even better, temporarily disable MySudo entirely.
I imagine the app needs to run in the background so that you can receive SMS and phone calls. Think about stopping the MySudo app from running like being somewhere without cell coverage, or with having your phone on airplane mode. Incoming phone calls will go straight through to voicemail, and SMS messages will be queued until you next connect to the network.