Jellyfin on the firestick. Remove internet access from the stick (LAN only).
Alternatively, DNS block amazon communications (Pihole, adguardhome, etc).
Edit: you can also install adguard, or other ad blockers directly onto your firestick.
Edit 2: there are also a number of ad preferences burried deep in the settings. My fireTV has no adblocker, but only displays static banner ads on the homepage.