Hopster is a bit tricky to block. It works by running a SOCKS proxy server locally and tunnelling all requests through HTTP to Hopster's servers. Therefore if your corporate allows web (HTTP) access, Hopster will work.
1. If you've got control over your users' desktops, you could lock them down to prevent the installation of unauthorised software.
2. Hopster first of all makes a plain HTTP connection to an external server. You can monitor this IP to see if different ones are used and you can build up a list if necessary. Also, you can monitor the text in the connection request to see if content filtering is possible. You will need to install Hopster and use Ethereal to obtain the above info. You can then block the IPs at your firewall.