I sincerely apologize for the experience you've had thus far with the Go product, and I want to try and help. We've fixed a few notification alert problems that perhaps you have not pulled down in the app.
Before diving into the issues we've fixed, I wanted to say I'm unable to reproduce the problem you are having. If I use the Settings > Account > Your Notification Settings menu to disable notifications, I do indeed stop receiving them. Do you have a support case number I could go look over so we can perhaps get to the bottom of the problem together? In case you haven't updated the app in some time, there may be a few problems you are running into.
An issue existed on versions before 2.3.0 where it was not possible to change the alert settings, but this is no longer an issue.
There was a second problem where each device in the network would report a usage alert. This means that if a device was on the wireless network, you could potentially generate 3 usage alerts (one for the GR, GS, and GX). This was fixed in version 2.20.0 of the Meraki Go app and beyond.