I agree with Tys. I'm a firm believer in switching to full auto and dumping a mag on someone who won't acknowledge that they were hit. Once the mag empty and they still haven't called hit, reload, move closer and repeat until you are close enough to hear them complain about you over shooting.
Now I admit that there are times when I've been slow in noticing or reacting to a hit. We've all experienced those single shots that make it through the brush that most likely bounced off 3 trees before making a near inaudible hit sound. BUT, it's no excuse for those few people who you are shouting 'hit' at. I recall a few instances where someone got me and I yelled hit and put my gun/hands in the air. As I stood up to walk away, I get another burst to which I look eye to eye with the shooter yelling in a loud voice I'm hit already. (I taught Phys Ed an coached for years so I can project) Most times than not, these are nubs who are running highcaps and aren't even looking at the target they are shooting at. Winding as they shoot.
As frustrating as it can be, all we can do is to try and educate. If you approach these nubs all hostile and hot headed, they won't listen regardless of how old or mature they are. Cooler heads will always prevail. Pull them aside or ask the host to and explain the problem. Same goes for people not calling their hits.