Act hurt. Many pets get a bit rough now and then. Make a noise like you have been injured. Recoil. Pull back. Hold the damaged remains of your injured limb. Whimper. This is both a baby and not human. They do not understand any english, so try to overreact. Do this early. Do this every time to establish a recognizable pattern.
1. Stop play. Take the ball and go home. Sulk. If you stop every time and leave the cat unsatisfied, it will change the behavior. Maybe even develop some empathy early. If they come see you to ask what's wrong, whimper and hold your hand still.
2. If you feel like going back into play, be shy and timid about it. If the roughness happens again, stop and don't try again until tomorrow. I don't know how I turned the letters on after I started with numbers and did that manually, but we are using letters now. We have always been using letters. What are you talking about? Letters forever
3. Show us a picture of the cat. We all want to see pictures of cats. This is the internet, after all.

Kitten tips?