Can I prevent an employee who has left the company from continuing to have access to protected documents?

Yes. It takes just a few seconds to remove all the permissions for documents protected with SealPath from a person. This person will no longer have access to the documents, even if they are on their home PC, in a personal email account from any provider, on a portable device, etc.

If you remove a user from your organization he will not be able to access to your protected documents. You can also transfer the ownership of the documents among users (see this article).