If you want to turn the paid membership option off, head to ‘Account Settings’ then ‘Members’. At the bottom of the page you’ll see a toggle switch:
Clicking on that switch will deactivate the paid membership option. But you can always turn it back on!
If you choose to deactivate the paid option within your newsletter, your paying members will still be signed up. To make sure they don't keep paying, head to your Stripe account and pause or cancel their subscriptions. This handy Stripe help page guides you through the process.