I have incomplete payments showing up for my form.

I have incomplete payments showing up for my conference registrations. What's the best way to handle this? Is there a way to not allow them to submit their registration form without payment? Is there a way to send them an auto email saying their registration isn't complete?

I did check your form and found that you have added PayPal payment tool. Please note that when form is submitted, users are redirected to PayPal checkout page. If user do not settle the payment, that submission ends up in incomplete payments.

If you want to process the payment within the form, I would suggest you to please add PayPal Pro payment tool in form. This will require user to input the credit card details in form and payment will be processed on click on submit button. You may like to take a look at the following guide which should help you: https://www.jotform.com/help/458-How-to-Integrate-Form-with-Paypal-Pro 

Please note that Paypal Pro requires business type PayPal account.