I'd go back to BD's earlier suggestion but you need to actually DELETE the other account's data before a Secure Erase Free Space, which only erases data that has been marked as deleted
. It doesn't erase other accounts if they are still current. Have a look in the users folder and see if there is a deleted user account in there. If there is, put it in the trash and empty the trash
THEN do Secure Erase Free Space.
In System Preferences, Accounts, make a new admin account on your computer. Log out of the old and log into the new.
Select each other account one at a time and press the minus button to delete that user with the secure erase option checked (if it's available on your OS). My computer is running Mavericks, perhaps an earlier version of the OS may not have that option - if that's the case, just erase the disk from Disk Utility with the Secure Erase Free Space option once personal data has been trashed and the trash emptied.
But the important thing about SEFS is that it only works on items that have been marked deleted, not on those that are still on the disk.
This method also leaves the installed system on the disk and the new admin account which, of course, will have nothing of consequence in it. While not a blank disk, it's good enough I should think. The new user can do what he will with it but there will be no data of yours left on it.