Can I remove other people from my PlayStation Plus subscription?
You may want to look into setting a passcode on your account. This is not a fool proof solution, but it appears to be the only one, as you cannot force the PS4 to ask for your PSN credentials when you boot up/log in.
- Go to Settings -> Login Settings -> Passcode Management
- Choose a passcode from the on screen prompts.
- You can also delete the passcode from here, but you will need the passcode that you set.
You will also need to stop the PS4 from auto-logging in to your account, as it will simply boot to a screen asking for your account's passcode. In this case, you will need to create a new account for your cousin to use. This should be effective in stopping your cousin from using your account.