Rosalind - 7:56 pm on Sep 20, 2012 (gmt 0)
You might change/rename your own stylesheet, and add the following to the one they're using:
background:url(DO_NOT_INFRINGE.jpg) right top no-repeat;
Where DO_NOT_INFRINGE.jpg is a picture on your own server with a message such as "this site is stealing our stylesheets. Visit my site instead", etc.