Normally when signed in on here go to Manage Account > My Memberships.
If it doesn't show an active Boost TV Membership with a cancel button option, then do you have another NOW account or are billed through a third party such has BT TV, TalkTalk, Amazon etc.
Forgot to mention if you do find the Boost cancel button, then make sure you cancel it fully by following the screen steps given (i think from memory there is around four).
Then go back to My Membership and it should say cancelled in red text underneath the Boost Membership.
You also usually get an email confirmation within three hours that the Boost has been cancelled on your account.