I call a method in a library which needs to receive XML. One of the fields in this XML is called "usercomments" where the user can enter free form text from the front-end. Some of our users have figured out a way of crashing the system.
When certain hexadecimal characters make their way to the database, the method in the library fails.

This is the kind of error that I receive when the field to the XML Parser.
PHP Code:
System.ArgumentException occurred
  Message='', hexadecimal value 0x13, is an invalid character.
  Source=System.Xml
  StackTrace:
       at System.Xml.XmlEncodedRawTextWriter.InvalidXmlChar(Int32 ch, Char* pDst, Boolean entitize)
       at System.Xml.XmlEncodedRawTextWriter.WriteElementTextBlock(Char* pSrc, Char* pSrcEnd)
       at System.Xml.XmlEncodedRawTextWriter.WriteString(String text)
       at System.Xml.XmlEncodedRawTextWriterIndent.WriteString(String text)
       at System.Xml.XmlWellFormedWriter.WriteString(String text)
       at System.Xml.XmlWriter.WriteElementString(String localName, String ns, String value)
       at System.Xml.XmlWriter.WriteElementString(String localName, String value)
       at Ax.Frameworks.BOF.VOBase.WriteXMLElement(gt db, XmlWriter xmlWriter, bz dbFldInfos, a0 p, Type type, BOFDbTblAttribute dbTbl, Boolean excludeKeyFields)
       at Ax.Frameworks.BOF.VOBase.ToXML(gt db, XmlWriter xmlWriter, Boolean excludeKeyFields)
       at Ax.Frameworks.BOF.VOBaseCollection.ToXML(gt db, XmlWriter xmlWriter, Boolean excludeKeyFields)
       at Ax.Frameworks.BOF.VOBase.WriteXMLElement(gt db, XmlWriter xmlWriter, bz dbFldInfos, a0 p, Type type, BOFDbTblAttribute dbTbl, Boolean excludeKeyFields)
       at Ax.Frameworks.BOF.VOBase.ToXML(gt db, XmlWriter xmlWriter, Boolean excludeKeyFields)
       at Ax.Frameworks.BOF.VOBase.ToXML(gt db, XmlWriter xmlWriter)
       at Ax.Frameworks.BOF.VOBase.ToXML(Boolean includeEncoding)
       at Ax.Frameworks.BOF.VOBase.ToXML()
       at AxPartyClass.AxPartyC.SetPartyContainer(String iPartyType) in PartyC.cs:line 233
  InnerException: 
Is there some kind of method in C# that will sanitize this and remove all the invalid hexadecimal characters from the text fields, so that I can do a cleanup of the comments field.

By invalid hexadecimal characters, I mean those characters which the XML parser won't be able to parse. I don't have a list of which characters break the library and which don't.