http://wikipewt4n75trifcse2ggaieot27svognl7idu6xmmqn37kf64rvsyd.onion/index.php/Intrusive_Surveillance
From here, under the "Erase" header, select Erase Free Space. This overwrites all the empty free space on your hard drive. However, if there is material on your hard drive that is unreadable by OSX (i.e. Windows Partition, etc.), this will remain untouched.