Private Elements (or Tags) are perfectly legal in DICOM, details of the structure of Private DICOM Elements can be found in DICOM Private Elements.
Removing all Private elements from any DICOM instance is quite simple, following sample code shows you how to do it in DicomObjects.
DicomDataSet ds = new DicomDataSet();
ds.RemovePrivateAttributes();
Dim At As DicomAttribute
Dim Index As Integer
For Index = Image.Attributes.Count To 1 Step -1
Set At = Image.Attributes.ItemByIndex(Index)
If At.Group Mod 2 > 0 Then
Image.Attributes.Remove At.Group, At.Element ' if odd group number, remove it
End If
Next