I'm finding that when a mail merge field is enclosed in parentheses that calling MailMerge.Execute(object[], object[]) is adding check boxes to the output document.
Reference "Really Simple Renewal.doc" as the input, "Completed Really Simple Renewal.doc" as the output and here is the code for the test case.
Dim doc As Aspose.Words.Document = New Aspose.Words.Document(inputDirPath & fileName)
Dim oPath As String = outputDirPath & Path.GetFileNameWithoutExtension(fileName) + ".doc"
Dim tags() As String = New String() {"PT4_Individual", "PT4_IndividualEntity", _
"PT4_IndividualEntity_SoleProprietor", "PT4_IndividualEnity_LLC", "PT4_IndividualEnity_PC", _
"PT4_IndividualEnity_PA", "PT4_IndividualEnity_Corp"}
Dim values() As Object = New Object() {"FishKill", "", "", "", "", "", ""}
doc.MailMerge.Execute(tags, values)
doc.UpdateFields()
doc.Save(oPath)