Umm, I would like to see this cancellation scenario happen when the IFR
student is on his check ride with either an FAA examiner or DER in the
plane... Watching two branches of the federal government duke it out
could be highly entertaining...

Anyway, if you are not truly VFR or if you need that IFR approach for
currency there is that word in the regs, "Unable", for a reason... A
single word, unadorned - and no further explanation will be offered by
me any more than he did. The controller is then obligated to continue
to handle you IFR...
Now, having said that, being normally a cooperative cuss, and if I am
just shooting the approach to stay sharp, and he is busy, etc., I won't
care, I'll simply grunt, "roger that, 57 pop", hit the 1200 button and
continue the approach...

denny