I don't remember my iCloud account
My step daughters phone was stolen and she doesn’t remember her email or password she used for iCloud. How can we lock her iCloud account so it isn’t used
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That's going to be an issue. She could try resetting her password at iforgot.apple.com but if she doesn't know all of her information to access her account, it's unlikely she can reset.
If she didn't establish a recovery key, she can follow this process: https://support.apple.com/en-us/HT204921 But, it will take quite some time to grant access.
Provided she has a strong passcode on her device there is little to worry about. Once she regains access, follow these steps: https://support.apple.com/en-us/HT201472