How to fully terminate EXCEL.exe from running tasks in office.interop?

I have to open an excel file and want to get the data to word a document. I use to this code:

private Excel.Application excelapp;
Type ExcelType = Type.GetTypeFromProgID("Excel.Application");
dynamic ExcelInst = Activator.CreateInstance(ExcelType);
this.excelapp = ExcelInst;

this.workbook = this.excelapp.Workbooks.Open(Filename : this.filePath, ReadOnly: true);

I used all the techniques to close/quit/dispose the opened process (except for Process.Kill), but those won't work. How do I fully terminate any running background tasks?