How do I add contacts to VerticalResponse from Wix only if the email doesn't exist?

I get users signing up on my WIX website.  I want a zap to add the email address to Vertical Response if that address does not exist.  If it does, stop the zap.  I’ve gotten as far as connecting to the two, creating a split for the condition that exists and does not exist.  I don’t know how to terminate the zap if the email does exist.  I think the condition for does not exist works, but I can’t test it.