sudo mdadm --remove /dev/md0

Then edit the /etc/mdadm/mdadm.conf file and comment out the RAID definition.

<file bash /etc/mdadm/mdadm.conf> #ARRAY /dev/md0 level=raid5 num-devices=3 metadata=1.2 spares=1 name=bionic:0 UUID=76c80bd0:6b1fe526:90807435:99030af9 # devices=/dev/sdb1,/dev/sdc1,/dev/sdd1

Also, edit /etc/fstab file and comment out the line that enables auto-mount of the RAID device.