To avoid unhautorized access to app when it’s running, i suggest to insert in project settings, a switch to allow password request when the user tip stop button (password is the same of the blynk user).
An image is worth more than a thousand words 
For such cases, you could always use project’s shared access, which would provide access to running project and disable it’s edit
@BlynkAndroidDev but shared access doesn’t restrict a user from pressing a button which I think is what @Dinosauro is looking for.
@Dinosauro you can do it yourself with a button and the terminal.
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In shared access there is no ‘stop button’ which would stop the project, project’s stop and run are only on the side of the master project.
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Reading the OP again I see it’s the project settings that he / she wants to restrict access to and not the buttons etc so sharing would work.
Not really a solution though because that gives access to your hardware and presumably the OP doesn’t want to do that.
I guess he / she needs to consider http://blynk.io/