There is a way to shut these spammers down. You just need to change a few settings in Skype.
On a Windows machine, open Skype. Click Tools>>Options. Select the Privacy tab. Select the button labeled "Only allow people in my Contact List to contact me." This will block all users except those you've added to your contact list. Those are the people you really want to talk to anyway. Click Save when you're finished.
The settings are a little different on a Mac. Click Skype>>Preferences. Select the Privacy tab. You'll see separate options for all of Skype's functions. Set each to "Only people in my Contact List." Close the window. Settings are saved automatically.