I think you can only limit this person's account if they are less than admin...you probably should not give this other person admin rights if you are not comfortable giving them full access to your computer.
More answers here on protecting your mac:
http://www.pocopico.com/rants/osx_hardening.php
Go to system preferences/accounts and you can limit what someone sees when logged into your computer directly. As for networking, I'm less familiar with changing that.