Block all sites except one
Solution 1:
Check out Parental Control. This allows to restrict applications to run as well as web sites to visit.
Alternatively you can use ipfw to restrict IP/Internet access:
sudo ipfw -f flush
sudo ipfw add allow tcp from me to xx.xx.xx.xx
sudo ipfw add allow ip from me to xx.xx.xx.xx
sudo ipfw add deny tcp from me to any
sudo ipfw add deny ip from me to any
To disable just run sudo ipfw flush again