I know this might be a bit of overkill but i got this to work in .net with it opening the excel file writing the value "Something New" and then closing it ( and destorying the Excel process in the task manager)
Hope this works for you
Dim Xl As New Excel.Application()
Dim Xlb As Excel.Workbook = Xl.Workbooks.Open("C:\Kk.xls")
Dim Xls As Excel.Worksheet = CType(Xlb.Worksheets(1), Excel.Worksheet)
TextBox1.Text = Xls.Cells(1, 1).value
Xls.Cells(1, 1).Value = "Something new"
Xlb.Save()
'Changed Code
exWorkbook.Close()
exObj.Workbooks.Close()
exObj.Application.Quit()
exObj.Quit()
exWorksheet = Nothing
exWorkbook = Nothing
exObj = Nothing
GC.Collect()
GC.WaitForPendingFinalizers()