Well for anyone looking for an answer - I found a clue in another post to check the comments related php files in the root skins folder. I edited both _item_comment.inc and _comments.disp to comment out the reference to avatars and Voila! Nice and tidy again.

Hope this helps someone.

...and one more little place - inside /inc/comments/model/_comment.class.php

function avatar( $size = 'crop-64x64', $class = 'bCommentAvatar', $params = array() )
	{
		//if( $r = $this->get_avatar( $size, $class, $params ) )
		//{
		//	echo $r;
		//}
	}

There is probably a better way to do this, but it keeps that nasty Gravatar from displaying on your admin pages as you clean up your comment spam.