Assuming you are trying to block a 3rd party I would go with Group Policy or Content Filter
Find the client from the Client Details Page - You can cick the plus icon on the right had side of the column headings and add MAC as a caterfory. If you know the MAC you can do a search of all the clients on that page.
thanks for your response, i have group policy set but the default policy on meraki is not working here in Nigeria my country, so i have so add the site one by one, else if you know a better alternative
If you need to apply a group policy in mass you can do it from the clients details page. Just check mark all the clients wanting to apply to. Click the policy button on the top left of the list and then select the Group Policy Radio button. Then choose the policy to apply to all the clients. Another option is to do it by VLAN if you want it to apply to all devices behind the MX.