In my manual, List Lightning, I describe how you want to have an exit redirect in your OTO page - that is, the first page that your Autoresponder sends visitors who have just opted in. And there isn't very many choices for redirect plugins in the Wordpress world... free, that is... the best that I've found is the 'Pinoy Pop Up On Exit' plugin - which is what I'm currently using. It can stop someone from closing a webpage, give them a javascript notice, and redirect them to your downsell page....