Of course it's possible. Remove/delete them from your account at https://my.roku.com and factory reset them before gifting them. Make sure to log in with the email address that's displayed on each one under Settings->System->About in case you have multiple Roku devices or you won't see them listed under the account. Note that once they are factory reset the new owner will require a working physical remote to set them up and get them connected to a Wi-Fi network. The mobile app won't work until they are connected.
How to factory reset your Roku® streaming device | Official Roku Support
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