I'm not so sure about /reconnect because if a hacker get's kicked out of a game, they can abuse this command. I'm pretty sure lots of rule breakers will.
"hackers can exploit it by rejoining"
Okay, imagine the situation, bear with me ok? Good.
You are fighting a hacker, suddenly he gets kicked. Yay!
Now your argument is that they will just rejoin and therefore this suggestion is not a good suggestion,.
However, have you considered that it takes quite some time before rejoining? First they need to click away the kick message, then they need to log in on cubecraft, then the lobby needs to be loaded, then they need to type /rejoin (or whatever command it will be), then the world needs to be loaded and
only then can they fight back again. All this takes quite some time. Time enough to hit the replacement entity (aka "mob") of the player a lot of times, probably killing or at least heavily damaging the hacker. Now you might say: "some hackers are fast with logging in so this won't work". Again, there is an easy solution: Just add a timer. Players need to wait a minimum of x amount of time after logging in before being able to run the command. This will prevent exploitation.
The point is, the hacker will lose either way, with or without this suggestion being implemented.
Pretty easy to fix.