There's rules for passengers and airlines... but enforcement is limited, because who wants to slow down boarding by checking.

And compliance is hard for passengers, because you have to call in to book the special case, and who wants to call in?

But theoretically, a passenger that will encroach on an adjacent seat can pay for the extra seat (I don't know if they need to also pay for seat assignment to get two seats next to each other), and then if the flight doesn't actually sell out, the extra seat fee is refundable. But when you actually board, people will see the 'empty' seat and try to sit in it, even though you paid for it. Etc.