grant no quarter
grant (someone) no quarter
To offer (someone) no mercy, concession, indulgence, or leeway. This match determines if we're heading to the finals of the tournament, so go out there and grant them no quarter! Our boss grants no quarter when it comes to the standards of our projects.
grant someone no quarterand give someone no quarter
Fig. not to allow someone any mercy or indulgence. (Originally meant to refuse to imprison and simply to kill one's prisoner.) The professor was harsh on lazy students. During class, he granted them no quarter.