If you need to cancel a free trail then it was not free
The trial is free, if you continue beyond that free trial period, then that product isn't free.
Part of the function of accessing the free trial is the completion of a payment mandate that will kick in at the end of the trial. That is the point at which the user decides if they want to proceed. Then they will have to ensure that they cancel that trial and uninstall the product.
Personally I would never sign up to a payment mandate prior to undertaking a trial.
Asyn has given a link to that means of cancelling, however I don't know if there is a cancel means within the user interface or the account.avast.com Subscriptions section.