BCNF decomposition does not prevent insert anomaly?
Edited

I think that the relation is in BCNF, but it is known that this fact does not always solve all the anomalies. For this reason other normal forms, like 4NF, 5NF, etc., for instance those based on elementary keys (Elementary Key Normal Form, Key-Complete Normal Form, etc.) have been defined. 

I am not sure of what other kind of dependencies are present in your schema, but I want to show a decomposition, as pointed in a comment below by @philipxy (correcting my previous version), to eliminate redundancies and anomalies from the schema.

polls (poll_id, question)
   poll_id → question
options_of_polls (poll_id, option)
   no non-trivial functional dependencies, so the key is (poll_id, option)
votes (partecipant_id, poll_id, option, stars)
   partecipant_id, poll_id, option → stars