Q: I can't close Skype.

Open the task manager and try closing it from there.

Can you force-quit it? Command-Option-Esc, select Skype, press enter.

And, of course, if all the above fails, you can force the Mac to shut off. Save ALL your other files, docs, etc., quit all other programs, tap on the Caps Lock so the LED lights up, then press and hold the power button till the Caps Lock LED turns off and all is silent.

To be on the safe side, I'd run Disk Utility and do a First Aid on the drive in case Skype left cruft behind. Details are OS-specific so if you need help, holler back with what OS X flavor you have.

Skype uninstall/reinstall