The real problem is not so much people using the loo, as a stream of extra people walking through and adding to the noise experienced.
At the end of the day, an airliner is a public transport vehicle, so one is not going to experience the solitude of a private jet, but if the airline has charged a substantial premium for premium accomodation, then it should use reasonable efforts to deliver that promise.
A word often used by airlines to describe their premium offerings is 'exclusive', which by its very definition means keeping others out.
Bottom line, if the accomodation (i.e. cabin) is sold by class, then it should be mainly restricted to the people who bought that class and the crew should be trained (and performance managed) to discretely enforce this.
I usually travel long haul business and would not dream of using the first class loos (or any other part of that accomodation, out of respect for the premium those pax have paid to enjoy a higher level of service and peace and quiet.)
With respect to Flaps Forty, if a cabin crew member cannot competently influence passenger behaviour in a discrete way, they should not be working in a premium cabin, not least because (sadly) one gets enough awkward characters in there, who can be very difficult to manage.
Sometimes, what should be a relaxing journey, turns into an ordeal because of the behaviour of some business class pax, but that is another matter, for another thread.
One final point, as a J class pax sleeping through the night, I really do appreciate cabin crew who master the art of walking lightly through the cabin and minimising the amount of time the curtain to the galley is open when entering/exiting
This really does make a big difference to quality of sleep.