Minecraft server crashed, what can you do?

Me and a friend own a Citybuild Server, which is a lot of work. However, we're sometimes hacked / crashed by people. In the console you can't see because it was spammed and the log was somehow deleted. We also have a registrar user who has rights. Do not know how he has it because we do not grant rights. He has a & k name which is why we can't ban him. Server is not a bungee record and is in 1.8.X.

I appreciate help.

Turn on whitelist. Only people you know. Online mode = True.

But that should be a public server