Are you still able to log into your Roku account at https://my.roku.com using the email address shown via Roku menu path Settings/System/About? If so you can 1) Change your password to one known only to you, 2) change your email back, and 3) set a PIN and require it to be used to incur any charges.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
For account & billing concerns, it may be more effective to get help from our Billing team. You can contact them here: support.roku.com/contactus then choose 'Questions about my account or billing' on the drop-down menu. They are the best ones to help you out with that matter.