The account is under my ex-girlfriend's name and I don't remember her email and password but it's trying to charge my card how do I cancel my card from being charged
Do you have the Roku? If so, Settings > System > About will show you the email address.
Even then, your best option may be to reach out to her regarding it.
Alternately, report the card compromised and get a new card with a new number.
Roku may be able to help, but think about the obstacles: you are asking about someone else's Roku account (if it's under her email address, it's her account). Imagine if someone else contacted Roku about your Roku account. See the issue? Not saying they can't help, but it will be a difficult and time consuming process. That's why I recommend the other two options. Roku may have some magic solution that @RokuDanny-R can offer, but I'm skeptical.
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
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If you have access to the Roku device, we suggest checking the Roku account email directly from the device by going to Settings > System > About. Once you verify the Roku account is from your former partner, we suggest that you contact your bank immediately to report the activity. They will best be able to assist you further with that process. We can only proceed on our end if we are able to find the associated Roku account or the Roku account holder.
For further assistance with account and billing-related issues, please reach out to our support team directly here for your 'Account or billing' issue, here: https://support.roku.com/contactus. They will be able to further assist you.