To completely remove a raid array it has to be stopped first before it can be removed.
mdadm --stop /dev/md0 mdadm --remove /dev/md0
mdadm --zero-superblock /dev/sda
When using RAID, such as RAID1 arrays, where identical partitions exist on both drives it can be useful to copy the partitions from sda to sdb:
sfdisk -d /dev/sda | sfdisk /dev/sdb
WARNING: This will dump the partition table of sda, removing completely the existing partitions on sdb, so be sure you want this before running this command, as it will not warn you at all!