One way hash of data records (PBKDF2?)
Even with PBKDF2, You wouldn't be able to use salt anyways, because salting relies on a one-to-one mapping between the salt and a user identifier (email). You cannot keep the email, so you wouldn't know which salt to use when a user comes back with his email. 

Simply iterating SHA 256 for a large number of times is a way to discourage preimage searching. Other (more solid) cost-incurring algorithms include scrypt. Although time-space balance attacks still work if you don't have salt to work with. (Using SHA, possibly inserting large constant data after input string to further slow it down).