Yes, as soon as the installation is done, you can cancel your contract with your current provider via letter, phone or mail.
Your contact numbers
Will you transfer your (fixed and/or mobile) numbers to Proximus? Don’t cancel them, Proximus takes care of the transfer.
The devices of your current provider
Are you renting devices at your current operator? Make sure to return these devices, as Proximus doesn’t support them. You will need a new modem and/or decoder from Proximus.