In some cases it would be good to know that your VMs don´t have an ISO-File or Host device configured e.g. if you want to vMotion some VMs to another Host. Here´s how you could solve this problem:

 
[vSphere PowerCLI] C:\Program Files (x86)\VMware\Infrastructure\vSphere PowerCLI> Get-VM | Get-CDDrive | 
Set-CDDrive -NoMedia -Connected:$false -StartConnected:$false -Confirm:$false