How do I prevent a form from being submitted by someone pressing Enter key before clicking on SUBMIT button

Submitting the form upon enter key press is the standard behavior of any form. Unfortunately it is not possible to change this behavior in our editor.

I would recommend you to enable the "Required" validation of your fields, in that way your form fields wont' be submitted empty:

Another option would be adding a captcha field: http://www.jotform.com/help/205-How-to-Add-a-Captcha

If you do not mind a bit of programming and are able to embed your form to a webpage, this action can be disabled with a bit of javascript.  Here is how to do so:

https://www.jotform.com/answers/355370-How-to-stop-form-submitting-when-enter-is-pressed