Go to your account page and uncheck the box labeled auto-renewing.

When your membership expires, your Slate Plus account will be deactivated, and you will no longer have access to any member benefits, including the ad-free podcast feeds. If you’d like to try Slate Plus again in the future, sign up here or locate the option to reactivate your membership on your account page.

If you signed up for Slate Plus in Slate's iOS app, follow these instructions to stop auto-renewing.