But you're never going to be able to totally eliminate "clearance not available", so there will always be a need for pilots to action.
I agree.
Saying "no" must be a tool that's available to controllers.
I agree.
Trainees get flustered & bugger things up & do things a rated controller wouldn't do. That's just the nature of training.
Say what?
Are you saying, in effect, that those who happen to be flying in the system, when a trainee happens to be behind the mic, just have to wear the consequences of the trainee's lack of competence? The training system allows aircraft to be the subject of ATC decisions that a competent controller wouldn't make?
Yer kidding, surely.
And, I'll make the point a final time: We don't know that in this specific case the trainee was flustered and buggered things up. If s/he was, the report should say that. For all we know, the trainee's supervisor said: Tell him: "'Clearance unavailable'. Ya gotta teach these pop-up nobodies a lesson."
If it was a report about an accident involving a student pilot who was flustered and buggered things up, the report would say that (in modern day euphemisms) and - hopefully - analyse why the instructor did not intervene at the point things started to form the shape of a pear.
How hard is it to train pilots that if they're intending to request a clearance that they need a contingency plan in case they're knocked back? You have plan Bs for every phase of flight - abort the take-off, avoid that large patch of cloud, go around to avoid that cow. How hard is it to ask the trainee pilot to explain & implement what they'll do if denied a clearance?
Not hard in theory, but sometimes people ask: Why would we be denied a clearance?
Now I can add to the usual explanation, which is usually already very surprising to punters: "There could be a trainee on duty who's flustered and buggering things up. They could say 'no', even if there's not another aircraft for miles, and the supervisor will just watch it all happen."
This is part of the reason for some private pilots having a fear of controlled airspace.