You may try to to use Worksheet.Cells.RemoveFormulas() method for your source worksheet in the source workbook before copying it to destination workbook. This will replace underlying formulas with calculated values in the cells. See the following sample code for your reference:
//Instantiate a Workbook object that represents the your source Excel file
Workbook _workbook = new Workbook("e:\test2\Sample.xlsx");
//your destination workbook.
Workbook workbook = new Workbook();
Clear existing worksheet(s) and add a new sheet named "Report" to it.
//Remove formulas by calculated results in your source sheet e.g. named "Report" which is to be copied to other workbook.
//Get your source sheet in the source workbook
Worksheet worksheet = _workbook.Worksheets["Report"];
//Copy from source to destination
//Save your destination file.
Hope, this helps a bit.