If you have installed Developer Tools on your Mac then you can delete individual aliases in the Finder sidebar, using Property List Editor, which you'll find in Developer > Applications > Utilities.
In your Home folder go to User > Library > Preferences > com.apple.sidebarlists.plist
Control-right click and choose Open With > Property List Editor.
Scroll down to "useritems" and click the grey arrow next to it to reveal contents.
Click the arrow next to CustomListItems and you'll see a list of entries marked "Item" with a numeric suffix.
Click the grey arrow next to the Item whose number corresponds to the sidebar alias that you want to get rid of and you'll see the info associated with that entry, including its name, as seen in the sidebar.
If it's the right one, click again on the Item header (i.e. "Item x") and hit delete.
Quit Property List Editor and Save changes.
Quit and relaunch Finder and the Item you've just deleted will be gone from the sidebar list.
(Note: if you haven't already installed Developer Tools then you'll find it in the Optional Installs folder on your MacOS X installer DVD.)