Get a firewall - something like
Zone Alarm Free. As long as this person hasn't already put any malware on your system, that'll keep them out. Even if they have put malware on your system, the firewall will stop it 'phoning home.'
Even better would be to get a router - most routers have a built-in NAT firewall that's even more effective than a software-based one. And, of course, you can then connect more than one machine to your connection.
Quite honestly, no-one should go online without a firewall - to my mind it's even more vital than anti-virus software.