How to prevent a specific user from submitting?

If an user selected no in a radio button to a question (are you a member), how do I prevent the user from submitting?

It is because the event is a member-only registration form and I wish to display a message or popup that they had to become a member to submit.

How could I do this? Thanks!

Hi Marcus

At the moment, there is no way to provide this restriction

Tuan

Okay, is there something like... Validation?

I want to do the same like the blank field. If the user enter blank, they get a certain message. How do I do the same for the radio button?