NotifyOnValidationError or ...?

Hi. I have the following.

<TextBox Name="txtCostoNeto" Text="{Binding Articulo.CostoNeto, UpdateSourceTrigger=PropertyChanged}"/>

public class Articulo
{
...
public decimal CostoNeto { get; set; }
....
}

1) If I write in the textbox NO numeric characters; signals an error in red. I can disable this check?

2) By default, the textbox has a number 0, how can I do so that the textbox is empty?

Thx!