If you are using a normal GridView then this is how you do it.
All you need is to find that control in each row then you can disable it
Protected Sub ddRolloverSource_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
For Each myDR As GridViewRow In GridView1.Rows
If myDR.RowType=GridViewRowType.DataRow Then
Dim FoundRadioButton As RadioButton = DirectCast(myDR.FindControl("rbPostTaxAll"), RadioButton)
Dim FoundDropDownList As DropDownList = DirectCast(myDR.FindControl("ddlPostTaxAmountOrPercentageF"), DropDownList)
FoundRadioButton.Enabled = False
FoundDropDownList.Enabled = False
End If
Next
End Sub
EDIT Added Bill's suggestion
EDIT Added solution for DataGrid
For DataGrid use this
Protected Sub ddRolloverSource_SelectedIndexChanged(sender As Object, e As EventArgs)
Dim RowCount As Integer = DataGrid1.Items.Count - 1
For i As Integer = 0 To RowCount - 1
Dim RowItem As DataGridItem = DataGrid1.Items(RowCount)
Dim FoundRadioButton As RadioButton = DirectCast(RowItem.FindControl("rbPostTaxAll"), RadioButton)
Dim FoundDropDownList As DropDownList = DirectCast(RowItem.FindControl("ddlPostTaxAmountOrPercentageF"), DropDownList)
FoundRadioButton.Enabled = False
FoundDropDownList.Enabled = False
Next
End Sub