User interface of scanner can be hidden only if you use TWAIN driver for your scanner.
WIA driver does not allow to hide (disable) user interface - it's limitation of WIA drivers when they are used via TWAIN interface.

Here is a code snippet for VB.NET that shows how to acquire images without UI: 
  Try
    VSTwain1.StartDevice()
    If VSTwain1.SelectSource() Then
      VSTwain1.ShowUI = False
      VSTwain1.DisableAfterAcquire = True
      VSTwain1.OpenDataSource()
      VSTwain1.UnitOfMeasure = Vintasoft.Twain.UnitOfMeasure.Inches
      VSTwain1.PixelType = Vintasoft.Twain.PixelType.RGB
      VSTwain1.Acquire()
    End If
  Catch ex As TwainException
    MsgBox(ex.Message)
  End Try