Hi Luca,
If you want to restrict users to not approve the payments you can restrict them from the view Manage my area work center approvals.
So the persons will not be able to approve.
and
Note that while creating approval process and setting the work distribution rules if you select Direct Approver, you must select an employee responsible by clicking Edit. On
the following screen, select one employees responsible so it goes to that particular responsible person only..
Hope this clarifies you.
Regards,
Suresh.