To reproduce:
- Initialize a RadDropDownList with some items and set one item Enabled property to false
- select the disabled property by typing its first letter from keyboard

Workaround:
- check for disabled items in the SelectedIndexChanging event.

void radDropDownList1_SelectedIndexChanging(object sender, Telerik.WinControls.UI.Data.PositionChangingCancelEventArgs e)
{
    if (radDropDownList1.Items[e.Position].Enabled == false)
    {
        e.Cancel = true;
    }
}