Some hacked clients come with a ''server crasher''. This can cause a game to lag intensely. All the players will have trouble moving around because of the lag.
I have seen a video of someone trying this on Cubecraft not that long ago, it made everything lag and players said this in chat too just like in your screenshot.
I don't know if these screenshots are valid proof though, it's probably better if you try to record the lag/crashing.
I would report the player for: Illegal client modification - other or maybe for trolling.
This should definitely be banneble.