if you format it or if you want it to completely disapear like the card is new you need Roadkil's DiskWipe, available from http://www.roadkil.net/
1. Download Roadkil's DiskWipe.
2. Extract the contents of the compressed file.
3. Open DiskWipe. If you are running Windows Vista or Windows 7, right-click the program icon and select Run as Administrator.
4. Select the drive to wipe.
5. Select the type of wipe to perform; DiskWipe can zero-fill the disk or write random data.
6. Enter the number of passes.
7. Click Erase to start the process.
8. At the end of the process, close the program. You can reuse the wiped disk.