1. Take a piece of abrasive paper, put in on a flat surface with the abrasive side upwards, put your disc on it, press the disc with your palm, and draw circular movements so to create concentric deep scratches. Radial scratches won't necessarily do the trick, they must be concentric.
In case you want to check, afterwards, that the disc is indeed unreadable, don't forget to clean it thouroughly to avoid damage to your drive!
2. Take a very small screwdriver, or a sharp knife, and try inserting it at the edge of the disc, between the two polycarbonate layers. Most discs will split in two halves rather easily. Easier with TY and MXL discs, more difficult with MCC, almost impossible task with Ritek.