An unobservant friend was staying at my house, we decided to make pizza. I told him to use the toaster oven, I came back later with some ingredients for the other dishes I bought and he had made pizza in my bbq. He put a marble tray on the grate and put the pizza on that, he was careful for it not to touch the grate, but did not know about zeiah. I know the pizza is not kosher probably but is the grill still kosher. It is an enclosed bbq that he occasionally opened to check the pizza. The Shulchan Aruch permits pizza to be made in an oven but I am not sure about a grill.
The grill needs kashering. Assuming that the bbq was not used for meat for 24 hours prior to making the pizza, the most practical way to kasher it would be to wait 24 hours from when it was used, and clean it well. Then put in a nice amount of coals so that he bbq will get nice and hot, heat up the bbq well, and then it is kashered.