How to remove network security group in resource manage

Hi, I use command: Remove-AzureRmNetworkSecurityGroup -Name  $nsg -ResourceGroupName $rg -Force | Set-AzureRmNetworkSecurityGroup, but gor error:

I guess the reason is because the networksecuritygroup associate with a subnet, I tried commandline:

Set-AzureRmVirtualNetworkSubnetConfig -VirtualNetwork $virtualnetwork -Name $subnet.Name -AddressPrefix $subnet.AddressPrefix | Set-AzureRmVirtualNetwork

but it does not work.

How can I remove the networksecuritygroup through script?