This is a typical requirement and should be there from the beginning. We often need to disable the submit button, based on business requirements / dependencies of other controls. We cannot use the new responsive designer Form for a new project because of this limitation and must switch to older Forms which is a pitty. Please, add this...
Wonderful! - Thanks!
We added the ability to use custom buttons in our recent release. This means you can now hide the default submit button and then use rules on a custom submit button.
Thank you for your feedback, I will now open this idea for voting.