I like what Jason Fried tried, making people pay $100 upfront, and refunding them $100 bills at the door:

https://world.hey.com/jason/5300-in-100s-3be4c194

https://world.hey.com/jason/5300-in-100s-the-verdict-74c4108...

Even a token fee like $5 would probably dramatically cut frivolous RSVP and noshow rates.

(I am sorta building something around this idea… lots of yak-shaving happening though.)