Pop up on Newsletter Form Page

Any chance there is a way to stop a user from navigating away from a page if they have not first submitted their info into a form?

I have a client that would like this function to make sure that new members are registered for their email list.

The new member is sent to an outside page to enter payment info and then they are redirected to a welcome page that has a constant contact form. The client would like a popup or something similar that says something to the effect of "Wait! Make sure to sign up for email updates."

Thanks!