I prefer using a HOSTS file which includes a large list of "banned" advertising and porn sites. It's also easy to add, or remove, any site you wish. All you do is download one of the free HOSTS files form the internet, then place them in the appropriate folder on the computer. If the other users aren't really computer educated, they simply won't be able to access the sites. Then again, if they are computer experts, they might know how to delete the file, so it may, or may not fill your needs. Read about HOSTS files at the link below:
What is the Hosts file?
There are also a fairly large number of third party "Parental Control" programs available which allow you to monitor and control all sorts of things on the computer..
The most appropriate way is to control your computer to such a degree that the kids lose computer privileges should they visit inappropriate sites. That's what I've done...Something to think about.
Hope this helps.
I am concerned that my children are accessing websites that they shouldnt be, i know the website addresses, how can i block these sites, many thanks