1) Open card door of camera
2) Remove memory card
3) Close card door
4) Place card on flat surface
5) Find a big hammer
6) Smash the card with as much force as possible. Make sure it breaks into many pieces.
7) Job done. Pics inside can NEVER be used again.
If you read carefully instead of putting down comments just because they don't spoonfeed you then you'll notice that there are many ways to do it - including also Photoshop. The mentioned link at CS homepage describes the steps for Photoshop. Searching the forum for 'watermark' will reveal more possible ways and links to the respective software.