I think the company should be well aware of that fact before doing business with them but nonetheless the company should still be allowed to do business with that client at their own risk but should not violate any human rights of course. Although there should not be a requirement to do business with them.
I personally do not think companies should be allowed to business with human rights violators. I feel that sentence speaks for itself. I am not speaking against a company's transgression, but more-so against the specific violators. If these people made the decision to violate human rights in general, they should lose their privilege of doing business with respected companies. If they capable of violating general rights, who knows what else they could be capable of?