matt varnish is the answer usually.
The problem is, that not all varnishes are equal. For example the VGC-Matt varnish is pretty satin, while the VMC/VAC ones are much more matt.
Also helps if you do multiple coats of them (2 or 3) diluted only a tiny bit to make it flow better (for example for AB) after shaking up the dropper really good.
If you can: Testors Dullcoat or Army Painter Anti-shine are some that really can kill the shine (the AP less than the Testors one).
Matt-medium: in theory it should also help, but in my experience with the Vallejo one: doesn't help. Then again other brands might do it correctly.