grid_center.ColumnDefinitions[3].Width.Value = 0;

<Grid VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Name="grid_center">
    <Grid.RowDefinitions>
        <RowDefinition/>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="200"/>
        <ColumnDefinition/>
       //изменить размер этого столбца
        <ColumnDefinition Width="200" Name="grig_col_200"/>
    </Grid.ColumnDefinitions>

grid_center.ColumnDefinitions[2].Width = new GridLength(0);

grig_col_200.Width = new GridLength(0);