Have in mind that by redirecting all hotlink traffic to that page you will use up more bandwidth than just the single picture they are hotlinking.
The page you redirect to is 367kB including all pictures. So it's just to calculate what the total bw used is if they hotlink 10, 50 or 100 pics. It adds up.
If bandwidth is a concern, use this RewriteRule instead:
RewriteRule .* - [F]
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