ah yes little brothers and mothers letting shit slide... I don't envy you. Glad those days be over!! But sorry for your troubles kiddo. Just another badge of honour for later life. I can't tell you how many times I felt like strangling my younger bro in his bed while he slept...

A) Why not take the poster from his room where it is and swap it out with useless paper. (I'm assuming it's rolled up in a corner)

B) Take it back and hide it for a few months until he lets it go

C) Cut it up in front him (He's doing it to annoy the shit out of you, so go bat-shit crazy. Dare him to test your patience...)

Damn that sucks - must be so hard to keep your temper