Here is a quick trick for disabling the save button when a specific choice field is selected. For instance, say that you have a set of choices. If a user selects a certain field you can disable the save button. The example below is a form with a set of three choices and the Save and Cancel buttons. As you can see the Save button is disabled because Option 1 is selected.
Now when we select Option 2 or Option 3 the Save button is no longer disabled and you are able to Save the form.
Now let's go and open up the form. You can see below that we have set a rule on the Save button. If you click on the FX button and then use the Contains runtime function like this: contains([Name Control], "Name of Option")
Then publish and you are good to go! That simple. See the two images below for reference.
And there you have it! A quick way to disable the save button on a form.