Hacker seems to know the ip address of the server even if we changed

There is a dude that keeps joining into our Minecraft server that I made for 16 of my friends and keeps changing his name, there is no possible way to ban him as he has a space in his name, I wanted to turn on online mode but that would also prevent alot of my friends who uses Tlauncher to join the server, there is also geysermc plugin so that it allows players to crossplay. Apparently the hacker is also using Tlauncher to access the server, no matter how much times we change the server's IP address he still keeps joining and destroying the server. Any suggestions as to solving the problem?

Whitelist won't work as my friends with tlauncher need to join, somehow the hacker can still bypass aternos whitelist