Hi,

I'm in search of a solution that gives to the moderator a possibility to show in the activity stream of a member, all content items excluding content that member posted in its own topics.

Is there a way to do this ? The objective is to be able to quickly see whether or not a member answers to other topics than its own.

Thanks for your help.

No, not in the Activity Streams.  You really can't exclude items created by a member.  

To accomplish your goal, I think another approach might be to view the users activity (go to his profile > Activity), and then simply compare his posts to the topics.