Disk Utility has a secure erase function.
- Open Disk Utility
- Select the disk that you wish to erase
- Select the Erase tab
- Click on the Security Options... button
- Move the slider to Most Secure
- Click the OK button
This screen shot is for Disk Utility, version 13 (517), that comes with Mavericks (OS X 10.9.5), later versions may vary in appearance:
This will securely erase the disk. As you have selected the most secure option, this may take a while.
This option states:
This option meets the US Department of Defense (DOD) 5220-22M standard for securely erasing magnetic media. It erases the information used to access your files and writes over the data 7 times.