Hi I have problem with a datagridview with combo boxes firing unwanted events handlers.

I have a datagridview with 4 columns all data bound. All of these columns have comboboxes. Column 2,3,4 has a custom generated combobox with code with on the 'DataGridView_EditingControlShowing event' and have event handlers (Dropdown,selected Etc). (Data in the custom generated comboboxes needs to be filtered and worked on)

Code:
 

Friend WithEvents ComboBoxPowerPlantSelect As System.Windows.Forms.ComboBox
    Friend WithEvents ComboBoxAttachment1 As System.Windows.Forms.ComboBox
    Friend WithEvents ComboBoxAttachment2 As System.Windows.Forms.ComboBox

Private Sub TblMachineCombinationListDataGridView_EditingControlShowing(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewEditingControlShowingEventArgs) Handles TblMachineCombinationListDataGridView.EditingControlShowing

        Select Case ColumnSelected
            Case 3 : ComboBoxPowerPlantSelect = TryCast(e.Control, ComboBox) 'generate the combobox
                MachinePowerPlantSelected()
            Case 4 : ComboBoxAttachment1 = TryCast(e.Control, ComboBox)       'Generate the combobox
                Attachment1Selected()
            Case 5 : ComboBoxAttachment2 = TryCast(e.Control, ComboBox)       'Generate the combobox
                Attachment2Selected()
            Case Else : Exit Sub
        End Select

    End Sub
The problem is if i select a column with a combobox (Col 2 or 3 or 4) that i have generated THEN select a column on the datagrid that i have not generated Col 1 the event handler (Dropdown) from the previous generated combobox is fired.
How do i stop this, or what is the best way of disposing of the previously used combobox that i have generated after the user has selected the data in the combobox.
I don't know where to put the ComboBoxPowerPlantSelect.dispose() method to handle this properly in my code

Cheers