Along the lines of what Joe already said, there are some applications out there. I'd recommend the following articles:
The first one will get you a list of things to try, along with sites to download them and get more information. The second is of special interest because it'll bring up some points you may not have thought about, including legal issues you may come across. At one point, I worked for a State University, and we were not allowed to filter or restrict web access as the state was burdened with enforcing first amendment rights. If someone complained about another person's use of the web, we had to move the complainant to somewhere they couldn't see the offender's screen.
This was to avoid a lawsuit against the school, but seemed counter-intuitive at the time. Of course, this rule only applied if the people were adults, so keep that in mind.
In your case (as you've answered that you a drugs and alochol counseling charity), I'd say you should get a filter that allows you to block specifically drugs and alcohol-related content in addition to "adult" themed content.