Is there a way to prevent the form to be submitted until payment is validated?

Next issue.  Our membership is $40.  A membership submitted online is only valid when payment is received.  Many times we receive membership applications with no payment.  Most of the time its because they are having a problem with paypal.  Is there a way to avoid submitting form until payment is validated?

If you'll use Paypal as your payment processor, then the only way to prevent form submission without making a payment first is by using Paypal Pro.

Complete guide: Difference-Between-PayPal-PayPal-Pro 

To sum it up, the main difference between a Standard Paypal and Paypal Pro account would be the checkout experience. Paypal Pro allows the user to enter his account information either on your website (where the form was embedded) or directly on your stand alone form.

This in turn will force your users to make the payment first, right on the form, before submitting it.