I should probably hack myself/spend a spoon to let people hug me rather than impose costly social rules on everyone else.

No. "Respect my boundaries" (in this case, quite physical ones) is not something that would count as a costly social rule that you'd be imposing on others. Enforcing an "only hug the people who want to be hugged" rule doesn't only help you, it also helps everyone else who might not feel entirely comfortable with hugs. And on a more general level, having a strict norm of trying to make everyone feel comfortable will make the meetups a better place for everyone who attends them.

I'm sorry to hear that you got unwanted hugs despite the sticker: organizers at future meetups should emphasize that the rules in the stickers should be respected.