//Cut, Copy and Paste are the common commands used any application,

<TextBox CommandManager.PreviewExecuted="textBox_PreviewExecuted"/>

//in above textbox code we can restrict these commands in PrviewExecuted event of CommandManager Class
//and in code behind add below code and your job is done

private void textBox_PreviewExecuted(object sender, ExecutedRoutedEventArgs e)
{
	if (e.Command == ApplicationCommands.Copy ||
    	e.Command == ApplicationCommands.Cut  ||
    	e.Command == ApplicationCommands.Paste)
	{
		e.Handled = true;
	}
}