Yes, they can refuse and it has happened before. Jim Kelly did NOT want to be drafted by the Bills but, when he was, they owned his rights. He spent some time in another, competing league (AFL?) before that folded and he went to the league.
Bo Jackson was drafted by Bucs in 1986 but told them he wouldn't play for them. They forfeited his rights and he was drafted by the Raiders the year after. Of course, he was playing baseball during that time as well.
So, TLDR, they'll own Bosa's rights and if they can't agree they'll most likely see a trade (like Elway with the Colts and Eli with the Chargers)