This is how you can disable paste in TextBoxes in WPF

MainWindow.xaml

<Window x:Class="WpfApplication9.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBox Name="MyTextBox" Text="Hello" />
    </Grid>
</Window>

MainWindow.xaml.cs

public MainWindow()
{
	InitializeComponent();

	MyTextBox.CommandBindings.Add(new CommandBinding(ApplicationCommands.Paste, Foo));
}

private void Foo(object sender, ExecutedRoutedEventArgs e)
{
	e.Handled = true;
}