You need to use a designer - I use the following to hide the
BackgroundImage property at design time.

namespace JGControlLib
{
    [Designer(typeof(JGMaskedTextBoxDesigner))]
    public class JGMaskedTextBox :
System.Windows.Forms.UserControl
    {
        //.... all the class stuff
    }
}

public class JGMaskedTextBoxDesigner : ControlDesigner
{
    protected override void
PreFilterProperties(System.Collections.IDictionary properties)
    {
        properties.Remove("BackgroundImage");
    }
}

The BrowsableAttribute is probably what you're looking for.