public partial class NativeMethods {

    /// Return Type: BOOL->int
    ///fBlockIt: BOOL->int
    [System.Runtime.InteropServices.DllImportAttribute("user32.dll",EntryPoint="BlockInput")]
    [return: System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.Bool)]
public static extern  bool BlockInput([System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.Bool)] bool fBlockIt) ;

}

public static void BlockInput(TimeSpan span) {
  try { 
    NativeMethods.BlockInput(true);
    Thread.Sleep(span);
  } finally {
    NativeMethods.BlockInput(false);
  }
}

编辑

添加了一些代码来演示如何阻塞一段时间