How can I prevent the ENTER key from submitting the form?

Often times, our users will fill out a field and then instinctively hit Enter, which then submits the form.  Then they have to start all over.  

Maybe a pop up window can come up asking if they meant to submit?

I'm afraid forms were designed that way (regardless if it's built in Jotform or not). This is something referred to as Implicit Submission as described on HTML5 specifications.

This is important to accessibility, assistive technologies and impaired users that cannot use a mouse at all. There are pages on the Web that are only usable if there is a way to implicitly submit forms, so user-agents (browsers) are strongly encouraged to support this.

If you really want to suppress this default behavior of forms, the only way around it with is to do it outside JotForm. Just get your form's Full Source Code, deploy it on your website, and use Javascript to prevent the Enter key from submitting the form.

Complete guide: How-to-get-the-Full-Source-Code-of-your-Form 

I found a really good read about this topic on this post in case you're interested to know why it was designed that way. At the latter part of that post will be alternatives on suppressing it (especially the Explicit Prevention part where they gave an example JS code snippet to prevent it).