To be more specific, if an admin is unable to remove a host from a VDS, one of the reasons could simply be that the host's VMs are still connected to the switch. It could also be the case that the VMkernel adapters are active.
But could the issue also be that the host assigned to another VDS?
A host can be connected to a max. of 16 distributed virtual switches.
In order to remove a host from a distributed vSwitch, all connections need to be removed/migrated. Please remember that VM's with snapshots - where one of the snapshots has been attached to the vDS - is also taken into account, i.e. such snapshots need to be deleted