Block All External Contacts in Skype for Business
1. Navigate to the Office 365 Admin Center at https://portal.office.com/adminportal
2. Expand Settings from the Left navigation
3. Click on Services & add-ins
4. Select Skype for Business in the right pane from the available list of service. This will open an additional settings menu on the right side of the page
5. Turn Off the toggle next to Let people in your organization contact people with Skype for Business in other organizations under External Sharing.
You can also take a blacklisting or whitelisting approach by blocking certain domains or blocking all domains EXCEPT certain whitelisted ones.