R6400v2 Respond to Ping on Internet Port

I've been getting a LOT of Smurf attacks lately. The research I've done shows this is a spoofed ping. I went into the Admin on my router and the WAN setup. It shows the "Respond to Ping on Internet Port" as checked. No matter what I do, I can't uncheck it. I want to disable this function. Does anyone have the solution?