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. After this, only Premium Members (the ones who have funds in their account) can see your Skype ID.
You can also select the minimum amount that a Member must have in his account to be considered Premium.
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.