There is the need to remove a child server from the CMA, however even the deregister command with the -force option does not remove it, giving the error:
Unable to Deregister Child from CMA. Failed to deregister from cma :: One or more Integration Service is associated with Cluster.
Login to the CMA, go to the General tab then choose Cluster.
This should show the IS Cluster that references the child server that needs removing.
Delete the IS Cluster and the child server should now be able to be deleted.