Hacker able to be dead and griefing--and override adventure mode areas

A hacker/greifer on the server that i play on is using a client that allows him to build/destroy (grief) in areas in which he should be in adventure mode due to command blocks. His player is dead but he is invisible and able to walk around. you can hear his footsteps and swimming sounds. he claims to have make the hacked client hes using, called kneesnap.He says this hack only works on 1.8 vannila servers. he discovered this because his client allows him to make a potion that kills players on creative mode, and on bukkit servers they cant respawn. making a cure for that was how he discovered this hack.