Memo to self: read more about George Bernard Shaw, playwright, vegetarian and curmudgeon extraordinaire, who knew how to decline an invitation and put some teeth into it. Here's how he turned down Lady Randoph Churchill when she invited him to a large society lunch:

If I refuse an invitation in conventional terms, I am understood as repudiating the acquaintance of my hostess. If I make the usual excuses, and convince her that I am desolated by some other engagement, she will ask me again. And when I have excused myself six times running, she will conclude that I personally dislike her. Of course, there is the alternative of accepting; but then I shall endure acute discomfor and starvation... I shall have to dress myself very carefully and behave properly, both of which are contrary to my nature.
...Only if I can be of any real service at anytime, that is what I exist for; so you may command me.