Navigate to /Users/YourComputerUserName/Library/Application Support/Skype/
if you are on Lion or Mountain Lion, this folder will have been hidden by default. If this is the case, press Command-Shirt-G to open the 'Go to Folder..." menu, and input the path above.
Find the username you want to take off the login dropdown menu. delete the whole folder titled with that name. BEWARE, because this also deletes ALL CHAT HISTORY you may have saved on that computer. however, your chat history should still be available on Skype's servers.
Your Skype ID has now been removed from the dropdown menu. Along with any data associated with that Skype name.