Ok, so there's a sign showing two turnings, one with a big no-entry sign and the other saying M9.
Then there's a "No Right Turn" sign on the left and two No Entry signs at the junction on the "wrong way" slip road, so there's certainly adequate signage for anyone who's paying attention.
They could add a "NO ENTRY" sign painted on the road at the entrance to the junction.
But, to be honest, what I think would probably be best would be to narrow down the entrance to the junction. Yes, ok, it might cause a few more tailbacks on the slip road leaving the motorway (I don't know how busy it is), but taking out that big wide entry would make it a lot less tempting for someone to turn into it.