STUBBORN
Impervious Obstinate
Impervious to argument
He is not the least bit clever
He was perversely slow to——
He is too stupid to make a blunder
As impassive as a statue
He is inflexible to all entreaty
He has been inexorable to all invitations
The victim of his own prejudices
The dupe of his own stupidity
Empty of the kernel of good sense
He made a plump refusal
It is like trying to use a large screw-driver to turn a small screw
Altogether obtuse
Not a grain of novelty in it
The zealous stupidity of amateurs
Prodigiously stupid
He is as stubborn as hickory
He turned a deaf ear to——