Don't want user to be able to "Upgrade" to lower subscription

Please look at the attached screenshot (first one). Notice how my members are given the option to "upgrade" to the free subscription? What I'd like to have happen is that free members see the option to upgrade to the paid sub (which is indeed happening), but paid members don't see the option to "upgrade" back down to the free subscription. I've also attached the config for my paid membership in case that helps. Thanks!