QoS doesn't prioritise a device but rather the type of network traffic a device uses. For example, someone playing an online game can have a higher priority than someone who's just browsing the web. QoS is also dynamic, so if the web browser suddenly jumps into an online game, they'll suddenly be given a higher priority.
What you want is bandwidth throttling based on a device's MAC address but you should only implement this if you pay, and therefore own, the Internet service, otherwise people are going to be pissed at you.