Skype for Mac
To only allow people in your contact list to call you or send you instant messages:
Sign in to Skype.
In the menu bar, click Skype > Preferences... and then click Privacy.
In the section Allow calls from, select Contacts.
In the section Allow messages from, select Contacts.
To block and report a nuisance contact:
In the sidebar, right-click the person you wish to block and select Block [name of contact]… A message box displays prompting ‘Are you sure you want to block [name]?’
Tick Report abuse from this person to help Skype block abusers and spammers in the early phase of their activities.
Click Block.