The same thing that stops people from getting access to your domain registration and changing the IP. You have a contract with your provider (ISP or DNS) which says that you own that IP/Hostname.

Your home IP address changes, but most business or commercial accounts are given a static IP address (or blocks of IP addresses) which never changes.

I think in this case it'd be the user not putting in any sensitive data or downloading executables to run from an internet radio.