A URL filter needs a list of URLs of undesired web content. Such a list is referred to as a URLFilterDatabase.
There is a free database that can be downloaded
from the website of the
University of Toulouse.
A large commercial database is available from
URLfilterDB.
See ufdbGuard and SquidGuard.
One page links to URLFilterDatabase:
lib/main.php:944: Notice: PageInfo: Cannot find action page