Slow Internet Connection HELP!?

[Adam Osman]

Okay so, lately I have been having really slow internet connection and I'm pretty sure it's my cousin using it as I can see in the "Network". I Cannot change the password because HIS mom said to not and if I do change it he's going to tell HER. So, I've tried telling him to stop his DOWNLOADS which is the main cause of my slow internet connection but he does not listen and I need to find a way to keep him disconnected from my Internet because it's unfair that I get to use the slow side and I PAY FOR THE INTERNET.
I've tried upgrading the internet and he is just eating it all. please help?