Sources of videos, music and pictures can be added easily using the GUI in XBMC on Raspbmc. However, if you want to remove some of the sources there is no way to do that in the GUI.
To do this:
Open the file
/home/pi/.xbmc/userdata/sources.xml and you will see the source entries in it. Remove or edit the entries you want and save the file. You can also add entries here if you want.
Restart XBMC in Raspbmc. The sources you removed will be gone.
Tried with: Raspbmc 1.0-hardfp-b20130208-u20140401