The problem is that the targets aren't rigid enough, and as a result, they get pushed back when hit, springing back acting like a catapult.

For really obnoxious targets (like the greed target to the left of the slot on TZ, which will frequently launch the ball over the flippers, even with a new PBR reinforced target), you have to get really extreme to fix it. I rebuilt mine with a quarter inch piece of steel as the back, and a really hefty angle bracket. That was sufficient to prevent it from sending the ball airborne most of the time.