You cannot remove the array from the domain in offline status. You will have to power on the array and then only you can remove the same from the domain being in engineering mode.  To remove an array from a domain, follow these steps:   
So I powered down a system (NS40G), removed it from our environment etc.  I can't figure out how to delete the system from Unisphere.  The Remove Button is NOT available.  Thoughts?  Damian
How do I remove system from Unisphere after power down?