I think they are suggesting that you change your WIFI password. This would block your former roommate from connecting to your network locally.
I'm not so sure that it will prevent them from connecting to your WM100 over the internet.
Also, changing your WIFI password will require you to perform a factory reset on your WM100 (per the instruction manual) and set it up all over again. Not to mention changing the WIFI password in all of your connected devices.
I suggest that you email X10 support with your question at: support.x10.com
They will need your name, your email address, your order number (if available) and a description of your problem.
Sorry I couldn't offer an easy solution.