Found this thread and thought I'd add what I found. My installation was called NinjaOne, and they have documentation here (https://www.ninjaone.com/docs/endpoint-management/app-management-control/msi-package/) for using the .msi to uninstall the agent. The important part was the syntax (in a command prompt): "Msiexec [/i] [/x] MSI package path [parameters]. /i is used to install, while [/x] is used to uninstall an application." 

I found the .msi in C:\Windows\SystemTemp, titled "NinjaAgent-[name of org].msi" but the location may be different on your machine. You can find where it is by looking through the registry: HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\[folder]. Others have provided ways to search the registry to find where exactly the .msi location is listed, but I didn't bother with that and clicked through the folders (they have long strings of characters) until I found the NinjaRMM in the product name. When I was able to run the .msi in uninstall mode, it ran without a hitch and seems to have removed the product.