What should I say to my friend?

Three days ago, my friend James invited me to go to a fire at his house. I told him I couldn't go because I was hanging out with my friend Mac, and I was not going to ditch my plans with Mac. For the three days leading up to the fire, James woud not stop bringing up me not going to the fire, and proceeded to leave comments on my FB about it, IM me about it, and text me about it. He told meto bring Mac, but he and Mac are not friends and Mac did not want to go. Last night, when me and Mac were hanging out, James and his sister texted me almost 30 times about coming and how everyone wanted me there, and called me 11 times (I answered none of the calls). He's been a complete jerk about it, and I know he is going to keep bringing up me not going. What do I say to him, while maintaing a friendship?