All you have to do this is to log into your account, go to Settings ⇒ Public Profile ⇒ Settings & Privacy.
You will find there this option: "Show Skype ID to public". You will need to select "No" and after that click the "Save" button. Afterward, only Premium Members (the ones who have at least 1$ in their account) can see your Skype ID.
You can change the minimum amount that a Member must have in his account to be considered Premium (1$, 5$, 7$ etc.)
IMPORTANT: Please note that the Skype ID is required for the members to be able to contact you on Skype and have calls with you.