Teleport hack??

There was a guy that made all hacked clients players teleport to himself for how long he wanted to how is something like this possible and is there a way to prevent it?

Decompile the client and remove the corresponding command.

It's always a good idea to take a moment to skim through other posts before making a new thread. I hadn't heard of this but there is a thread in the RotMG Discussions forum that attributes this to an old, uncompleted feature in CrazyClient that is still around in the most recent version. The most recent release of CrazyClient has been taken down while the issue is fixed and I'm sure Crinxii or someone else is already working on it.