Build 4.5.0.34 of xlsgen adds a boolean for switching on and off the xlsgen pivot table rendering engine introduced in 4.5.0.30. By default, it's on, which means xlsgen automatically calculates and renders the pivot table on save and on print/preview renderings. And should your application need to disable it, here is what you would do :

pivotTable.Options.AutoRendering = false;

before inserting the pivot table in a sheet.