To turn off renewal, visit your Slate Plus account page. You can get there by selecting Account, in the right corner of your browser. (If you're not already signed in with your Slate Plus account, first click the Sign In link to sign in.)
Within your Account, go to the Subscription & Payment page, and toggle Auto-renew subscription to OFF.
This will turn off your membership at the end of its term, and you will not be charged again. Your access will continue for the rest of your paid period, After that, you will no longer have access to the ad-free or exclusive versions of Slate podcasts and will only be allowed access to a limited quantity of articles.
If you signed up for Slate Plus in Slate's iOS app or in Apple Podcasts, follow these instructions to stop auto-renewing.