There are a few different options to go about creating a One-Time Payment and it varies depending on your Auto-Renewal and your Membership Level settings.
If you wish for all of your subscriptions to not be on a subscription plan, you can set your global renewal settings to "Never" in Restrict > Settings > General.
If you wish to manage subscription renewals on an individual subscription level basis, then the Custom Renew Pro add-on can provide this functionality and can override your global settings.
Membership Level Duration
If you wish to have a One-time payment with lifetime access, set your Membership Level Duration to "0". After the initial payment, renewals will not occur as the Membership Level is set to an "Unlimited" duration.