Block communication between 2 subnets

Hello,

My fortigate has 2 IPs: 192.168.1.1/24 and 192.168.2.1/24

Clients are set up in these two different subnets with their corresponding gateway (192.168.1.1 and 192.168.2.1).

How can I isolate these 2 subnets to block traffic between them?

I know it's not the proper way to isolate subnets but I need it for now. Thank you