BBM. Which is precisely why the "measles parties" are not just about fun and fevers, but demonstrate appallingly reckless behavior. It's the whole "reckless IDGAF about anyone else" part of this that will spur prosecution, and new laws IMO. Education clearly isn't working, so that's one of the next steps, IMO.
As I said before, another potential strategy, besides excluding the unvaccinated from daycares and public schools, is to take measures to severely limit the travel possibilities of people who "choose" not to vaccinate. Make proof of vaccination a legal requirement for all types of driver licenses, passports, and purchase of plane tickets, leaving exemptions for valid medical deferments.
Somehow, we have to get the attention of these reckless people and persuade them to change their behavior. It isn't, and shouldn't be, a lawful "choice" to intentionally spread diseases that can permanently harm or kill other people.