Killing Excel instance

Hi Guys,

I'm having trouble trying to kill an instance of excel from my app. This is the applicable form code....

Imports Microsoft.Office.Interop.Excel.XlDirection
Imports Microsoft.Office.Interop.Excel.Constants
Imports Microsoft.Office.Interop.Excel.XlLookAt
Imports Microsoft.Office.Interop.Excel.ApplicationClass
Imports Microsoft.Office.Interop.Excel.XlFindLookIn
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Public Class frmElement
    Inherits System.Windows.Forms.Form
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#Region " Windows Form Designer generated code "
 (blah, blah, blah....)
#End Region
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    Dim xlApp As Object, ws As Object, wb As Object, fnd As Object
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    Private Sub ComboBox10_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox10.SelectedIndexChanged

        Me.ComboBox11.Items.Clear()
        Me.ComboBox12.Items.Clear()
        Me.ComboBox13.Items.Clear()
        Me.ComboBox14.Items.Clear()
        Me.ComboBox15.Items.Clear()
        Me.ComboBox16.Items.Clear()
        Me.ComboBox17.Items.Clear()
        Me.ComboBox18.Items.Clear()

        fnd = ws.range("1:1").find(Replace(Me.ComboBox10.SelectedItem, "D_", ""), , xlValues, xlWhole)
        If Not fnd Is Nothing Then
            Dim r As Long = 2
            Do While ws.cells(r, fnd.column).value <> ""
                Me.ComboBox12.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox13.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox14.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox15.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox16.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox17.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox18.Items.Add(ws.cells(r, fnd.column).value)
                Me.ComboBox11.Items.Add(ws.cells(r, fnd.column).value)
                r = r + 1
            Loop
        Else
            MsgBox("Method could not be found. There are a number of possible causes for this...." & vbCrLf & _
                    vbCrLf & "1) A Method name was manually typed into the Dropdown box." & vbCrLf & _
                    "   Do not do this. Select a Method from the dropdown list instead." & vbCrLf & vbCrLf & _
                    "2) A method was deleted after creation of this batch." & vbCrLf & _
                    "   Re-select a method or re-create the missing method.")
        End If
        fnd = Nothing
    End Sub
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    Private Sub frmElement_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load
        xlApp = CreateObject("Excel.Application")
        xlApp.visible = False
        wb = xlApp.workbooks.open(AppPath & "Methods.xls")
        ws = wb.worksheets("Sheet1")
    End Sub
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    Private Sub btnDone_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDone.Click
        ws = Nothing
        wb.close(False)
        wb = Nothing
        xlApp.quit()
        xlApp = Nothing
        Me.Close()
    End Sub
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    Private Sub btnCancel_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnCancel.Click
        Me.btnDone_Click(Nothing, Nothing)
    End Sub
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But it doesn't seem to want to Quit the Excel application. Any ideas?

Cheers,

Wayne