I want to stop (UNLOAD) ispconfig3 from memory. Then, if I want to use it, I prefer to start it manually. I also do not want it to load automatically at server startup. How can I do this?
Are you saying you don't want apache to load? Because ispconfig is simply an apache virtual host.
Haven't looked at ubuntu for a while but
update-rc.d -f httpd remove
might do it.
However, if you're saying you want website functionality but you don't want the ispconfig site, you could investigate removing the symlinks for ispconfig vhost in /etc/httpd/conf/sites-enabled.
You would have to place them back each time you wanted ispconfig and then reload apache.
To my mind, not worth it. If you're thinking along the lines of memory usage as you have a VPS with possibly limited amounts of memory then maybe, look at the httpd modules loaded, do you need all of them - by disabling some of them you may an increase in memory.