Where are you, TidyDancer?! I need to know what you've sent by why of reply?!
I would go with something like:
Dear Mad-as-a-Box-of-Frogs Gluezilla,
I feel you are rather missing the point. Whilst I could understand being left out of your wedding due to numbers or budget, it is spectacularly rude of you to have sent me a Save The Date card and then not invited me. Contrary to your interpretation of their function, STDs are intended to forewarn invitees of the planned date before such details as the venue have been finalised. You have upset many people with this particular faux pas.
To add insult to injury, you have then asked me give up my time and arrange childcare so that I can help you decorate your venue in order to surprise your chosen guests; those whom you felt were worthy of an invitation. Despite my initial refusal, you have pressed the matter, clearly demonstrating your total lack of manners and etiquette.
To clarify, I will not help with any wedding preparation. Neither will I tolerate any further discussion from you on the matter. I wish you well with the wedding and your marriage, but I do not wish to hear from you again.
Regards
TidyDancer