Assuming you're using Synaptic, simply open synaptic, go to repositories (it's under a pull-down menu - I don't recall which as I don't have admin privs on the machine I'm working on right now), then uncheck the "CD" option - probably at the bottom of the window. Once you do that, be sure to click the "Reload" button on the main synaptic window, then you'll no longer need the CD.
If you're using apt-get or aptitude, edit /etc/apt/sources.list & either remove or comment out the line that references the CD/DVD, then update the repository. With aptitude, use the 'U' key, I forget how to do it with apt-get, but the man pages should help you.