Serious Calling Skype Group

You will have to create a Moderated Skype chat / P2P-based group.
Do this by entering the following command to any of your Skype chat windows /createmoderatedchat
Now you'll have a Moderated Skype chat, in which you will be able to use these commands listed under "P2P-based chat commands and roles"
https://support.skype.com/en/faq/FA10042...-and-roles
This will allow you to have much greater control over what's going on in your group. I can not remember if Moderated Skype group by default prevents calls to the whole group so that's something you'd have to test yourself. If it does not do it by default, look for a solution via commands i provided above.
Only downside to this is that you will have to reinvite all members in the group, as Skype currently does not support changing regular group to moderated one.
If you have any further questions, let me know.