When end-users have an unpaid invoice, they will be presented with an on hold screen that looks like the screenshot below.
This view is designed to encourage remittal of payment for continued service. Unfortunately, there is not currently an easy way for end-users to "exit" or cancel their subscription.
An end-user can change the url of the current view, which will end in /on_hold, and change that to /sign_out, if they wish to sign out.
Alternatively, if they wish to cancel their subscription, they can change the /on_hold to /dashboard to access their dashboard.
This can be complicated for some end-users, so site admins can assist with subscription cancellation by navigating to the user's profile and cancel their subscription on their behalf.
If an end-user does not remit payment within 30 days of the unpaid invoice being issued, then the on hold screen will automatically be removed, and the end-user will need to re-subscribe if they wish to regain access. You can learn more about invoicing here.