Generally, to use the Xfinity Stream app, a user has to enter the account login and password. Would someone in your household have given that to the person who made the purchase? You could change your account password and control who has it for streaming. There are also Parental Controls in the app where a PIN can be assigned.
How do I Sign out everywhere from Xfinity Stream?
I combined your two posts on same subject to the one that already has a reply. go to xfinity.com and sign in. go to account, then users. do you have users listed there? (have to go through these steps to assist properly).