How do you respond?

You stand up for yourself

It takes you a moment to react, taken aback by the blonde's brazen assertion that you were responsible for the collision. You were rotating on the spot and she was the one walking, so the situation is at least ambiguous, if not entirely her fault. The knowledge provided by the Rulebook tells you that in the event of a dispute, one would first normally consult an independent witness and here there is one readily available, as the older lady is still on the bench and looking up from her tablet at the two of you. However, she would be expected to add a punishment of her own. If you were still unhappy, you could instigate a full investigation, but the guilty party would get a severe judicial caning at the very least, so very few cases ever end up like that.

In this case, your bottom is sore, you still have a pending punishment, and you have a date planned; taking an unjustified spanking, no matter how quick, is not on your agenda.

"I'm sorry," you reply, more politely than you feel this girl deserves, "I think you'll find that you bumped into me."