well. not impossible. To your point, if they are ip-banned soneome then that person can just use VPN or Proxy to get around it. And if you use an email signup system then yes it would be more tedious but still not impossible so therefore only the try hard players would do it.
One way you could fix this issue is to add a token id to every client. So when I log in my clients token id is used to determine what my name is for that month. So for example token 54321 is florida's client. And if I tried to log into FloridaAlt it would check my client's token and say something like "please use account associated with this client". Then maybe the 1st of every month the token attributes (the name they are attached to) get reset, and the first time you log in that month is the name you are forced to use. I'm not sure how cncnet is currently setup but I've seen this system work before with ToonTownRewritten's client when they had similar issues.
The only way to get around this would be to uninstall cncnet and reinstall it to get a new token, but then you would lose your first name, so no point in really doing that unless you just want to restart the month for yourself And since the cncnet files are like a virus intertwining with your cnc:ra2yr folders/files, its actually a pretty tedious process to fully uninstall and reinstall anyway.
Just some food for thought