Hi, there are many work arounds for this, it depends on how you've sent up your program.
If when you sent out the first email, you changed their program status to "invited" or "sent", you can send a reminder email to all people who are still in that program status. This will work if when they do fill out the form, you update their status again to "registered" or "responded" or something similiar that states they filled out the form.
Otherwise, if you sync your form submits to a CRM, you can use that as a filter as well - if person is not yet synced to the campaign in your CRM, send them the email again.
But I agree with Mark, the easiest and most straightforward way would be to add a trigger that says "Has not filled out form" (I tend to add a 2nd filter on the program name in Marketo just to ensure the trigger is working properly).
Hope you sort it out!