How can I tell him we're strictly friends?

It urks me how guys can be so cocky thinking they'll eventually pursue the girl. Currently, there's this guy who I see just as a friend but either he likes me or he's just a flirt in general. He does brag about a lot and in my eyes, he really ain't sh*t lol so I just stare at him when he tells me these things and act neutral, its funny to see him disappointed that I'm not impressed. However, he continues to do things that insinuate he wants to pursue me some way. I've already stated that we're just friends and made small comments that should tell him we're just friends but he doesn't get it. We are going to his friend's birthday party and I think he thinks I'm going as his date. I'm going as his friend and I for sure don't want his friends to think we are something or I'm the new girl in his life. So I want to tell him something like "hey so I wanna make sure your friends don't think we're dating or anything, we're just friends." But I feel like that makes me look like I'm being controlling or bad in some way. But if I tell him in an unclear way, he might not get the idea. How can I just let him know we are not going as an "item" ?
How can I tell him we're strictly friends?
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