Someone is hacking/griefing my Server

If you’re running a vanilla server, you can turn on whitelist. If you’re running a spigot server, you can install plugins like worldguard to protect areas like spawn, you could also install a land claiming plugin so other players can clain their base and don’t have to worry about their builds being grief. For those hacking, you can /ban the player, for hackers you should probably /ban-ip.

turning off online mode will enable hackers to connect as anyone they want, so keep online mode on in your server properties.