I failed to locate a specific command to allow/disallow ICMP netmask replies.
However you can setup ACL's to filter either type 17 code 0 (request, which is the preferred since you normally want to block the original request and not waste any system resources on a reply which will be dropped anyway) or type 18 code 0 (reply itself).
Like so:
rule 1234 deny icmp icmp-type 17 0 destination 1.2.3.4 0
Where 1.2.3.4 is the ip of the router you wish to protect, or you could just drop any address mask requests like so:
rule 1234 deny icmp icmp-type 17 0
For more info:
https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol