Twice, players have joined my server and within maybe 10-15 seconds crashed it. It had immense lag in the last couple of seconds, and I'm not sure what hacked client they are using to do it, and, especially, how to fix it.

Here is the console log since the player joined in:

[Sun, 6. Feb 2022 20:03:18 GMT INFO] UUID of player WillPlays2021_YT is ba0d38d3-58ed-4a2a-9923-28644f18e764 [Sun, 6. Feb 2022 20:03:18 GMT INFO] WillPlays2021_YT[/185.73.95.155:38778] logged in with entity id 6897 at ([world]10.5, 3.0, -6.5) [Sun, 6. Feb 2022 20:03:22 GMT INFO] [Member] fullwoodenshovel: hi [Sun, 6. Feb 2022 20:03:22 GMT INFO] WillPlays2021_YT issued server command: /ad usehaxhere JOIN: usehaxhere.minehut.gg FOR NO AC - ANVIL DUPE - AND ABSULOOTLEY 0 BANS
[\{_}20:03\{_}]
[Sun, 6. Feb 2022 20:03:34 GMT WARN] Can't keep up! Is the server overloaded? Running 6726ms or 134 ticks behind
 
Then a couple seconds later, the server crashed.
Sorry, there is barely any information to work of here, but I am pretty sure it is the player causing the crash because both times, the player has stayed exactly still in the spawn position, although did move with the physics of Minecraft, since the spawn is one block above, and they were on the ground, and it happened seconds after the two players joined.
 

After this, I followed him to another server, and saw that he was staying still in the spawn spot, did fall and take damage, but didn't take knockback damage when we hit him. Shortly after this, the server crashed.

Thanks,
fullwoodenshovel