Are you asking if data can be encrypted so that only authorized people can read it? If so, the answer is yes -- we do it all the time. You're doing it right now when you view this web page using SSL. There are a couple of caveats, of course. You need to use a strong encryption algorithm, and you need to have a sufficiently long key to prevent a brute-force attack. You also have to insure that the sender and receiver have exchanged keys and that no one else has them.
If you're asking if the spy can send 100Pb of data and not be detected, that's an entirely different issue. It depends on how well the network is monitored, and how much that traffic differs from normal traffic. The "enemy" may not be able to read the data, but if they detect that someone is sending a large amount of encrypted traffic to a "foreign" country, it may be detrimental to the health of the spy.