Exclude thumbnail image from certain category

Hi there

I am looking to exclude the featured post thumbnail that is displayed in posts from one particular category though I wish to appear in all other categories. I am using a child theme based on Elegant themes’s Chameleon. The category concerned is ‘artists’

The index page is here
http://79.170.40.240/surreysculpture.org.uk/artists
and clicking on any pic/title there gives an example of the problem.

I believe I need to add:
‘<?php if ( in_category(‘artists’) && has_post_thumbnail()) { ?>’
to the single.php file but then get stuck after that as don’t know how to make the thumbnail not appear.

If anyone could help I would be extremely grateful.

thanks
Kevin