$excel = [Runtime.InteropServices.Marshal]::GetActiveObject("Excel.Application")

$sig = @"
[DllImport("user32.dll", SetLastError=true)]
public static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);
"@
Add-Type -MemberDefinition $sig -Namespace User32 -Name Util -UsingNamespace System.Text
$p=0
[User32.util]::GetWindowThreadProcessId($MSExcel.HWND, [ref]$p) |out-null
Get-Process -id $p | Stop-Process