For instance:

# update-rc.d -f apache2 remove

The “-f” is required if you have existing scripts in /etc/init.d/apache2. If you are planning on manually starting apache, the “-f” is [barring heroic/quixotic effort to create alternatives] a requirement.
Otherwise, in this situation, you will see:

update-rc.d: /etc/init.d/apache2 exists during rc.d purge (use -f to force)

A good write-up is here:
http://www.debuntu.org/how-to-manage-services-with-update-rc.d