Did you try using a after-startup microflow, added to your project settings, to retrieve the MxAdmin user and set
a default your desired password in the microflow?
You can change the password and username in the security settings of your app.
And you might need to perform some steps from this doc: https://docs.mendix.com/refguide/administrator/#221-free-apps
I'm not sure if it is possible to disable the admin for apps. But since it says that the MxAdmin is not created automatically using a data shot without it might disable it?