I would suggest reaching out to the school district in order to review and discuss the access level you have associated with the parent portal. The school should be able to provide further insight into your access level and assist in removing other children from your account.
Thank you for reaching out to us on the PowerSchool Community!
Only the school or district has the permissions to remove guardians from a student account. Unfortunately, due to security reasons we do not have the access to view/manage student settings or student information, which is why we request you to reach out to the school district to have the student removed from your account and disable any further communication from the school or district.
If the school says that the student is removed from your account but you are continuing to receive communication. I suggest verifying with the school if the form of communication i.e. your email or phone number is not added as the primary contact on the student's account.