A disk cannot be removed directly from an array, unless it is failed.
NOTE: This step is not needed if the disk is already in a failed state.
mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1
Both of the above actions can be done in a single step:
mdadm /dev/md0 --fail /dev/sda1 --remove /dev/sda1