How to close all Presentations that are running in the Background - WPF PowerPoint Interop

Hey,

i have 5 Views/Pages. In each one of it there is a Webbrowser Control that opens a  different PowerPoint file via Navigate(path) Method. After i Switch the page i want to close the last PowerPoint who was Opend in the other (the last) page so it wont take Memory unnessecary.

i get Access to PowerPoint interop like this:

using PPt = Microsoft.Office.Interop.Powerpoint;

//...

private PPt.Application oPPApplication = null;

so when i want to Close the PowerPoint like this:

oppApplication.ActivePresentation.Close();

it Closes but still runs in the Background. How can i Close it in the Background too?

Thanks in advance :)