The Palo Alto Networks firewall can block access to Apple's iMessage on iOS devices. Since there is no specific application in the applipedia database, this is accomplished by blocking the relevant URLs in the URL filtering profile and calling the profile in a security policy.
1. Go to Objects > Security Profiles > URL filtering and click Add.
2. Give the profile a suitable name and add the below URLs in the block list with the action selected as "block" and click OK.
3. Call this newly created URL filtering profile in the relevant security policy, click OK and commit the changes.