This is not griefing, not even close. Everyone can get a glider to get up there, I'm assuming everyone who has been on Timeless isle for more then 15 minutes has at least 1 glider, I always have a stack of them. It´s easy to get up there, there are 3 ways to get on there, going to the sea fly up and use glider and land there, jumping off the cliff from Cinderfall and land there easily and 3rd one to get a taxi from albatross airwaves. It's a dick move yes, griefing or against the ToS, no it isn't. If you're opening tickets for this then you're just wasting GMs time nothing more, he wont ban anyone or give them a warning, not ever.
Also here's a tip to avoid this: leave these people be, the reason they do this is because people get angry and pissed, that's the only reason, and the more hate and threats you do the more they do it and will do everything they can to piss you off. So pretend nothing happened, get a glider go up there get your kills and move on, they will stop.
Also would like to say he spawns at that spot, so yeah killing him where he spawns? That's not breaking ToS. It's like going apeshit people who kill Garnia cause he/she spawns where you need a glider/albatross to get to so is that "Griefing" too? Nope it isn't.