| For each virtual machine do the following: |
From the vSphere Client, right-click the Virtual Machine and go to "Edit Settings".
Select the USB controller, click the circled "X" to remove it, and click "OK".
From a PowerCLI command prompt while connected to the ESXi host or vCenter server, run the following command:
Get-VM "VM Name" | Get-USBDevice | Remove-USBDevice
Note: This will not remove the USB controller, just any connected devices.