Okay, what would happen if you just didn't do it. Are you prepared to live with the chaos for a while. Could it be that you are partly to blame for this? That, in your doing so consistently that they have come to expect it.

Also who is footing the bill for this? THis comes to mind too.

All last year I reluctantly helped out and even ran some events for a charity. I have learned now not to RSVP and not to arrive until the last minute when things get sorted out or to not go at all. I think that there are things you might be able to draw from this. Namely, Keep yourself busy with other things until the last minute. Make yourself unavailable to do these tasks.