lucy24 - 10:40 am on Feb 18, 2013 (gmt 0)

and because nothing is showing they don't even see a problem!

:) Since nothing is showing, they're not benefiting from their hotlink either. You'd think by now someone would have noticed that the image doesn't show up. But maybe not, if it's in the original offender's browser cache. (Don't they know that you should always test links by emptying your cache and also trying a different browser? Honestly, hotlinkers these days.)

By the usual yawn-provoking coincidence, it can't be more than 48 hours since someone else had a completely unrelated question that also involved garbage being attached to the end of an URL. If you wanted to, you could cut to the chase with (note, no anchors!)

RewriteRule \.jpg.+ /nohotlinks.png [NC,L]

If all your malformed jpg requests are from hotlinkers and similar unwanted sources, you wouldn't even need a condition.

Alternatively, if your existing hotlink routine includes an element like
\.(png|gif|jpe?g)$
you could simply remove the ending anchor. That's assuming for the sake of discussion that you don't have any normal pages with ".jpg" sitting in the middle of the URL.