This guide shows where to cancel Vidéotron (Mobility / Internet/TV) in Canada, with links to official sources.
Vidéotron’s help page states that you can’t cancel online and that you need to contact customer service to cancel a service.
They also note cancellation fees may apply and equipment return charges can apply if equipment is not returned.
Ask for a cancellation confirmation (reference number or email) and keep it until your final bill is settled.