Deleting book formats created when sending to device.

Is it possible to stop calibre from keeping formats it creates when sending books to my kindle?

For example, if I have a book in epub format, calibre needs to convert this to a .mobi format and send the .mobi format to the kindle. But it also keeps the .mobi format around which isn't needed.

I am trying to reduce the size of my calibre library or alternatively to stop it growing so quickly, but rather than deleting books, I was wondering if I could get rid of these temporary book formats which I don't need.

Also is there any way of detecting these books so I can delete them manually. I don't want to delete all mobi books, nor assume that just because there is another format, that I want rid of the mobi format.