How do you determine how easy it is to leave before you actually need to do it? Before even signing up?
It ranks hundreds of websites from "easy" to "impossible".
1. delete all your posts the same way they would be deleted if you went to them and pressed the delete button.
2. Anonymize them (replace user name with "deleted user <randomnumber>").
3. Keep storing the username
After that, delete all other data (profile, settings, e-mail, password hash, ...)