You don't say what avast version you are using ?
In avast free you haven't got many options, you could try to block the URL to Skype; avastUI, Additional Protection, Site blocking. But if the connection to Skype would use https, that idea goes out of the window as it is the Web Shield that actually implements this blocking and it doesn't monitor https traffic.
So you are looking at using something else, firewall or HOSTS file to block Skype.