guy keeps joining with my friends' names

That's what happens with cracked servers ¯\_(ツ)_/¯

Best solution would be not to use cracked,

or use a login plugin.

Install this plugin : https://aternos.org/addons/a/spigot/19362 or dont make your server cracked.

Quote from Mrlelz

this kid was banned today, not happy about it and kept joining using my friends minecraft usernames to bypass whitelisting please help btw ip banning doesnt work, console says ip is banned but he can join without vpn

If you have forge/fabric installed on your server, I can't help. But, if you have Spigot installed, install the opeNLogin plugin. Your friends need to type /register <password> <password> and enter the server fast, or else, the stranger could make register passwords and your friends wouldn't be able to play on your server. If that happens I recommend reinstalling the plugin.

Quote from Happy_AlexRO

If you have forge/fabric installed on your server, I can't help. But, if you have Spigot installed, install the opeNLogin plugin. Your friends need to type /register <password> <password> and enter the server fast, or else, the stranger could make register passwords and your friends wouldn't be able to play on your server. If that happens I recommend reinstalling the plugin.

Your friends also need to put strong passwords and remember them when they enter next time. First time you enter the server, it asks you to register, but when you enter next time, you need to login by typing /login <password>. I think you know how the plugin works.