All that God has to do to harden people's hearts is to remove the restraints. He gives them a longer leash. Rather than restricting their human freedom, He increases it. He lets them have their own way - RC Sproul

"All that God has to do to harden people's hearts is to remove the restraints. He gives them a longer leash. Rather than restricting their human freedom, He increases it. He lets them have their own way . . . It is not that God puts His hand on them to create fresh evil in their hearts; He merely removes His holy hand of restraint from them and lets them do their own will"

RC Sproul 
(Sproul, Chosen By God , 145).