If you've revoked the access of an admin to your Regpack account, they will NOT be able to access your organization's data.
A deleted admin can enter their credentials into Regpack on the login page and receive a security code email when logging in. However, since their access has been revoked, they will be notified that they don't have access to your organization or projects. 
Any Regpack admin account can potentially have access to multiple organizations in Regpack - that is why a revoked admin still has the option to login.
The Regpack system has security measures in place to ensure admins will only have access to projects they have permissions too. Once the initial two-step verification happens when the admin logs in, the system checks to see if they have access to any organization connected to Regpack. If they have access to another organization it will automatically log them into that organization. Otherwise, it will alert them they don't have access to any Regpack account. If you have any questions about admin permissions and access, reach out to Regpack Support.