How to respond?

I was just out on my deck watching the kayakers paddle by when a young women got excited about my SMU flag, smiled told her friend in another kayak that there was an SMU flag, and then gave me a "Hook 'em Horns" finger sign.
I was aghast but politely smiled as I didn't know how to respond appropriately (especially since my USC flag is also prominently displayed).
What is the proper response if I encounter a similar situation in the future?
I was aghast but politely smiled as I didn't know how to respond appropriately (especially since my USC flag is also prominently displayed).
What is the proper response if I encounter a similar situation in the future?