Sean - who plays Quidditch captain Oliver Wood - made the statement last week on his website.
But the revelation has annoyed fans so much that petition websites have already started popping up across the web.
One of the reasons is it could threaten the existence of Quidditch in the third movie.
There are three Quidditch matches in the third book - but it would seem to be difficult to show those without the Gryffindor team captain.
Fuming fans have e-mailed CBBC Newsround's website two online protest petitions so far.
On those two sites alone, there are more than 5,500 protest signatures begging the film's makers - Warner Bros - to put him back in the movie.
No good!
Many have threatened to actually boycott POA if he is not included.
One user, AveryJ, says angrily: "I will NOT see this HP movie if they take out the best parts! We need Quidditch and Sean!
"No Sean=NO MOVIE! Fans across the world want Quidditch in the movie!"
And that's a threat which is being repeated by hundreds more protesters.
Earrinde claims: "The movie's no good without Wood!"
And on our own message board users can't understand the decision either.
Winky writes: "It's a terrible mistake and I hope the director will understand that and make a few changes."
Warner Bros declined to comment about whether Oliver was definitely not appearing in the movie.
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