I tend to give people a bit of leeway when it comes to tagging my mobs, or watching which mob i have targetted and the they instant cast something on it. But after 5+ mobs like that I get a bit aggitated and /s "You realize that kill stealing / greifing is against the TOS right?"

depending on how they respond I'll either report them or chat with them for a bit. I've noticed that generally people are testing to see if you're a bot or not.