Zone hoarding usually refers to players outside of acceptable level ranges killing bots in a zone.
Say you're in Uber and he comes in and tells you to leave or he'll kill him. Give him his three kills then start your leveling like normal. If he kills you again, that's over killing and he's in trouble.
If he starts leading the bots out and killing them in the main room so you can't attack the bots without dying, he's zone hoarding.
If he shoves you into a bot, or the bot into you, that's also a kill on his part.
Once he's had his three kills, he's done. He can't 'attack you to low hp but not actually kill you.' Many people have tried to do that ridiculous thing.
He might start talking about how he 'gets a revenge kill..' for such and such reason. The only thing that triggers a revenge kill is if you try to mug him or attack him. Very unlikely.
He knows what will happen if he breaks a rule, so force his hand. He's bullying you out of the zone by threatening to kill you. Force his hand and let him max out his kills on you. After that, he can't do anything to you. Well, he can mug, but there are pretty easy ways to avoid that. Carry a few 'vendor trash' items with you, gladiuses, claymores, etc, and when you think he's going to try mugging, put your leveling items (anchetssword, boots/paws/wws) into a pack nearby. Once he takes his 2 trash items off you, get your stuff and go back to leveling.
I very recently leveled an alt from rl0 to 90, with these same players, and worse. I know their tactics and excuses. Your best defense: screen shots.