How can I catch my roommate stealing bandwith?

I rent a house with 4 other people and we all pay for the internet. We have a problem with one guy stealing bandwith on his playstation to the point none of us can use the wifi while hes connected. I've checked QoS and it doesn't look like its on right now but we have caught him setting himself as priority in the past. Is there any other way he could be setting himself as a priority or slowing us down? Also when I check connected devices on my wifi it says his status is blocked on one of his devices connected to that network. I'm not sure if that's anything but hes doing something to the wifi it's obvious but we can't figure out how to catch him and put a stop to it. He just plays dumb when we call him out on it. Please help thanks