I'm having a party, and someone i don't want to come is attending...?

Me and my group of close friends sometimes have these 'hang-out-at-someone-elses-house' sessions on weekends. One of the guys invited this girl called Kat without asking me. (It's at my place this week.) I don't like her because she flirts MASSIVELY with this guy, and really makes it awkward for pretty much everyone else. This sounds a little bitchy, but it really stuffs up the dynamic. I don't really want to just tell her she's un-invited, but what can I do? I don't really like her that much...