May be you have your Skype account shared across several devices, and want to log out from all those except the one you want to use.
Here's a simple method. I tested this in my PC and mobile device, so it works.
Go to your device and log-in to your skype account (say your Mobile phone skype account). This is the device now you want to use and want to log out all the other devices Skype accounts (say your home-pc skype account, office-laptop skype account etc.).
Go to any of the contact in your list and then go to your chat mode (basically go to your chat input).
Enter this command and press enter.
This will log out all your other Skype online accounts except the one you are currently entering this command.