1. The Holy Roman Emperor Lothair I.
Reason: if your party is held during his lifetime, the cost of providing extra security will be prohibitive. If it is not, you will either need to mess with historical timelines or host a corpse, which is a bit of a downer.
2. The dwarf planet Pluto.
Reason: it will not fit in the door.
3. Don Quixote.
Reason: He is a fictional character, and thus unlikely to attend.
4. The North Sea.
Reason: you will be too busy mopping up after it to enjoy your party.
Reason: there are too many capybaras in the world to fit inside a reasonably-sized party venue, and if you only invite some of them then the others will be jealous.
6. The platonic ideal of the colour blue.
Reason: if it vomits on your carpet you will never, never get the stain out.
7. The norovirus.
Reason: it is a terrible conversationalist, for one thing.