This article describes how to block Anydesk traffics without UTM configured then you can block it using ISDB.
Scope
FortiGate.
Solution
Step 1: Go to Policy & Objects and select Create New.
Step 2: Destination -> Internet Service -> filter the name anydesk and select Anydesk-Anydesk.
Step 3: Configure other fields and select OK.
Step 4: Drag the policy to the top to take effect.