Stateless Form Validation - Cancel Submit

Stateless Form Validation - Cancel Submit

I have a stateless Guest form (which I have embedded in my website).  I have logic that the requires the user to enter the guest names based upon their input in a Number of Guests fields. I need to ensure that they have completed the fields before they submit the form.

Apparently the cancel submit function does not work for stateless forms.

Number of Guests:3
Guest Name 1: John
Guest Name 2: Tim
Guest Name 3:

In the example above, I would like to alert the user that they need to complete all the fields (which I can do with an alert function) BUT I need to CANCEL the submission of the form.

Is this possible?