Layer 7 Firewall rules on the MR and MX will not be capable of blocking these messages for this use case. As @DCooper has already stated, have a look into Systems Manager where you'll be able to apply profiles to IOS devices to stop these messages/notifications. - https://meraki.cisco.com/products/systems-manager
Even better is Meraki allow you to create a Free Systems Manager network and try out the paid feature version for 30 days!
Eliot F | Simplifying IT with Cloud Solutions
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