In Word, the features you want to and can turn off are spelling and grammar checking, and rsid insertion.
This is for docx4j (a project I manage), not POI, but VariablePrepare shows you what needs to be done to de-fragment if you can't prevent it in Word. Since POI uses a similar XML marshalling/unmarshalling approach (albeit XML Beans, not JAXB), you should be able to convert that code to use the POI API.