To only allow people in your contact list to call you or send you instant messages:
- Sign into Skype.
- In the menu bar, click Tools, then Options.
- In the Options screen, click Privacy, then Privacy Settings.
- Under Allow calls from..., click the radio button for people in my Contact list only.
- You can also click the radio button for people in my Contact list only under the Automatically receive video and share screens with... and Allow IMs from... areas.
To block and report a nuisance contact, follow the procedure outlined here.