How can I cancel internet if the main user left the appartment?

Hi Kyotsuku

You need to contact your cousin and ask him to follow the procedure.

Remember you areresponsible of the modem and if you do not return it Fizz will charge you 150 + TX

Therefore, ask your cousin to cancel the contract, and to give you the information to return the modem.

1- Log into your account and go to Manage.
2- Click on Unsubscribe.
3- Scroll to the bottom and confirm by clicking Unsubscribe.
4- Tell us why you are unsubscribing.
5- Enter the date when you want Home Internet service to end.
6- Click Unsubscribe.
7- On the next screen, download, write down or print your return ID number and follow the instructions for returning your Wi-Fi modem.

Important: You have 15 days following the date you unsubscribe to return your Wi-Fi modem, or else you will be charged the amount of $150 (plus applicable taxes). Detailed instructions will follow.
How do I return my Wi-Fi modem? https://fizz.ca/en/faq/how-do-i-return-my-wi-fi-modem

source: https://fizz.ca/en/faq/how-do-i-unsubscribe-from-fizz-home-internet

Finally
https://fizz.ca/en/faq/what-size-box-do-i-need

Hi Kyotsuku,
This is why one should always order his own service. There are no cost for internet activation with Fizz.
If you cannot reach your cousin to cancel the service and return the modem, contact Fizz with this issue and see what your options are.
To do so, log into your Fizz account.
Make your way to the Solution Hub.
On the help page pertaining to your issue, click on the chat bubble located in the lower right-hand corner.
The chat service is available every day from 8 a.m. to 9 p.m. EST. The chat bubble won’t appear outside of these business hours.

this is a very good question, I wonder the same!

Good question. If you can't contact your cousin I would try to contact fizz directly.