Deactivating secondary accounts on Family account

[xray]

I have Windows 7 Home Premium Family Pack which allows three PCs to be activated.

Is there any way to make it so my PC is the only one getting to use it? I have reason to believe a not so trusty family member copied a disc and took a picture of the serial key and is using it on one or two PCs.

 

[Megahertz07]

Welcome to the Forum.

Activation is done by M$ servers every time you install with a valid license key. There is nothing you can do on your computer to block futures installations using your license.
You can ask M$ to block the key you have and ask them to give you another, but the not so trusty family member will continue to have his computers active till he needs to reinstall Win 7.
And if he already did the downgrade to Win 10 there is nothing you or M$ can do. Once Win 10 is activated on a MB it will gain a perpetual license.