Right now it's not possible to throw people out of the party. Why not build it like this. Whenever a rotten apple is in the group, the person who started the party can select that character and suggest to the others to throw that member out of the group. By means of a democratic process the group can decide to delete him or keep him. Everyone gets a dialog box with a yes/no option to delete that member. If 50% or more wants it, the guy/girl is history (so with a tie, he also goes). If the vote was no, it can't be suggested anymore. The human character gets replaced by computer controlled character (sort of henchman) with the same professions and level. Voila.
There could be variations, like that every member can suggest the removal of a character, but I don't think that's a good idea because those same annoying people will get the time of their life then

You can also decide if henchmen in the party are allowed to vote or not. Like, if they can vote, that they vote the same as the party starter. Not sure what is wise here though.
The only problem that I can think of would maybe be how to name that new computer character. I suppose all names must be unique in GW? But I'm sure ArenaNet has some very smart people who can solve that technically.
Somebody call ArenaNet, because I'm really getting sick of my missions
