To permanently delete your account, login to your dashboard, then click on the expand arrow next to "settings" in the left hand sidebar menu, then choose "account".
In the new view that opens, click on "Delete account", then enter the reason you've decided to leave (optional), your password, then click "I understand, delete my account and all my files" to confirm.
This will permanently delete your account, any active subscription you have with us, as well as all of your uploads. It cannot be undone.