Spoiler Alert: I don't think you should do much of anything about that guest, even if you feel like they are spoiling your party. People have all kinds of reasons for bringing special food, including allergies (which I hope everyone acknowledges as legit!), medical conditions (often invisible to the host's naked eye!), religious requirements (don't even mess with that, y'all!) and just plain pickiness (okay, yeah, but you still shouldn't be mean about it).
Most of us (I hope!) will make sure to have enough food for a vegetarian guest, and we can all eliminate an ingredient or two in the case of allergies. But if someone wants to bring their own and still enjoy your company, who cares?