Hi Peter,
I am really sorry for the delay of this reply as it took me quite some time to identify the real issue on your site.
It seems like the issue is coming from jomsocial itself where jomsocial will convert any user id that are "null" or "0" to current logged in user hence your comment guest avatar will always follow current logged in user avatar. Unfortunately to fix this issue we need to modify jomsocial core file itself or forcing guest avatar to always use easyblog default avatar instead of jomsocial.
I've applied temporary fix for you inside your /administrator/components/com_easyblog/includes/avatar/avatar.php to always make guest avatar to use easyblog default avatar. Hope these can resolve the issue that you are facing currently.