There is no perfect ban system and there will not be any.
User will always find a way to avoid a ban.
You have following options to solve that problem:
- Use a high security level on your voice server
- Put a password on your server and give it to the people.
- Put a password on your server and assign the permission to ignore it to your member groups. But don't tell anyone else the password.
- Remove the possibility for Guests to move out from default channel, to write in there etc. and to subscribe any channel
Edit
The Question how we could improve the ban system stands in the room.
Any idea is welcome.