Because any customer user in the portal can potentially have access to any customer (if granted access), we do not suspend users globally. If you no longer wish for a customer to be able to access one or more customers, just remove their access to those customers. If a customer user has no access to any customers, their login will fail stating they do not currently have access to any accounts.
Later on, the plan is to create a suspend feature specific to customers. But for now, effectively you can just remove/add the user's access to the customer. This action does not affect their login password or other settings on their user account. Adding the user account back to the customer can be done by clicking the add user button on a customer screen and then search for the user by email address.