What a difficult position to be in! God that you've got documentation of the insufficient progress.
Some suggestions. The first is harsh:
Have you actually said, in simple terms "Your progress is unsatisfactory. YOu are not capable of completing."
A left field idea
I suggest you ramp the pace up to make it around that which you'd expect from a satisfactory or better than satisfactory candidate. Your aim is to push the student - absolutely legally - into realising they are not up to the task.
Become a very very active supervisor, and set out a schedule of every two weeks a piece of writing, to take the candidate to a submission date.
Take notes of each supervision, and email them to the candidate & supervisory team.
Note the agreed task set for the next supervision, and note when this is not done.
And so on. Ramp up the pace to achieve satisfactory outcomes, and force the candidate to realise they are not meeting the standard.
Alternatively, what is the maximum period of registration allowed? Let them dawdle on, see them the minimum number of times you need to (at my place it's 10 supervisions a year) encourage them to submit, then let them fail.
That's kind of the nuclear option, but if your university won't support you to de-register the candidate because of unsatisfactory progress, you have to let them fail.