[Sennheiser]

Make sure the "only allow direct connections from contacts" option is enabled in your Skype settings. This will take care of most resolvers, but SpeedResolve has figured out how to bypass it. If you want additional security, you can configure Skype to connect through a proxy: https://imgur.com/a/NpFdW

speed resolve can grab skypes even if proxy is configured in skype.

or just use skype on a 4G Device and not your pc

[Mopex]

VPN or Proxies are your friend for this.

[Belgian]

Here is a tutorial on how to block Skype resolvers:
https://hackforums.net/showthread.php?tid=3726185

[Bapple]

Hey, you can also buy blacklisting on skype resolving sites. I know it may not be that helpful but It is worth a try