I tried using this
sudo /etc/init.d/apache2 stop, but it only temporarily works and the server starts working again after a reboot. Is there a disable feature or something similar?
To stop a service doesn't prevent the deamon to start the service on boot. For this you should deactivate the service to come up on all run levels
A simple command for this on raspbian is
update-rc.d apache2 disable
If you later want the webserver starts again on default just type
update-rc.d apache2 enable'
With the comand
sudo /etc/init.d/apache2 start you bring up the web server on demand.