Can you disable automatic mdadm startup?
/etc/mdadm/mdadm.conf
# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
DEVICE partitions
what about scan something like /dev/null ? I mean some devices without superblocks. So mdadm cant find arrays.
From a RHEL 6.7 setup adding AUTO -all in /etc/mdadm.conf to disable all auto-assembly, except for ARRAYs/DEVICEs otherwise specified, seems to do the trick.