You want to use your proxy server as a content filter, forbidding requests to certain places.
Use ProxyBlock in the httpd.conf to deny access to particular sites:
ProxyBlock forbiddensite.com www.competitor.com monster.com
This example forbids proxied requests to the sites listed. These arguments are substring matches; example.com will also match www.example.com, and an argument of example would match both.
If you want more fine-grained control of what content is requested through your proxy server, you may want to use something more sophisticated, such as Squid, which is more full-featured in that area.