Hello. I created a new itemtype and a new relation many to many for this new itemtype and products. So I need to avoid the shynchronization of this new item. Maybe I have to modify any property of syncjob?
You can go to the Sync you wish to run, click on it.
On the configuration tab that pops up on the bottom of the screen you can click on "Edit Configuration" button.
Then you can click on the item you do not wish to run the sync and toggle "synchronize".
Consider also checking on KBA 2987565 - SAP Commerce synchronization job and cronjob introduction, on the "ESSENTIAL and SYNCHRONIZATION ATTRIBUTE CONFIGURATION" part, as it too has the answer to your question.