If you run multiple grid infrastructures (aka RAC Clusters) on SuperCluster, which share the same set of Exadata Storage Servers (aka cells), adding ASM Scoped Security to the setup is a good idea. Even if there are no security reasons like multi-tenancy, just simply preventing accidental use of one cluster's diskgroups by another cluster should be reason enough to implement this simple precaution.
Of course, there is good documentation on this feature, available here. However, as often the case, the devil is in the details, so here's a comprehensive example of how to do this:
assign key for 'marsc1'='9e9a606a461a1abc6af43626e85af3b7'
list griddisk where asmdiskgroupname='DATAC1' attributes name
list griddisk where asmdiskgroupname='RECOC1' attributes name
alter griddisk DATAC1_CD_00_marsceladm04 availableTo='marsc1'
alter griddisk DATAC1_CD_01_marsceladm04 availableTo='marsc1'
alter griddisk DATAC1_CD_02_marsceladm04 availableTo='marsc1'
alter griddisk DATAC1_CD_03_marsceladm04 availableTo='marsc1'
# cellcli < script
list griddisk attributes name,availableTo
crsctl start crs
That should be all. You can easily verify that your other clusters can no longer see these diskgroups or disks from another cluster's asm: asmcmd lsdg --discovery
Now, repeat this for all of your clusters. The end result will be exclusive access to each cluster's disks, with no danger of intentional snooping or unintentional use.
One tool that comes in very handy for doing stuff on all cells at the same time is "cssh" - a one to many commandline included in recent versions of Solaris.