Hi Tushar,
As per your statement all Web Intelligence Reports are saved on same Folder where you have Dashboards. So please follow the below steps.
Step I: First apply "Folder Level Security" and Set only View to all User & Group
Step II: Now apply "Objects Level Security" on each Web Intelligence Reports and set "Noaccess" to all Web Intelligence Reports.
Hope this will help you as well.
Thanks,
Daya