How to disable submit button?

You cannot disable the submit button, but you can use the Show/Hide Condition to hide the Submit button based on a condition that you have set. For example, you can set a condition that if a field has been filled, the submit button will be hidden from the form. This way, the user won't be able to submit your form. You can check out this guide for more info: How to Show or Hide Fields Base on User's Answer

Here's an example:

You can check out this demo form to see the condition in action: https://form.jotform.com/62201232326945 
Regards.