The problem has worsened since the Win 10 Anniversary Update. At first, Outlook would not let me save calendar entries if the stub had been present when I opened the program; all other functions seemed OK. Lately, if I start the program without killing the stub, Outlook sometimes says it can't open files, or something like that. And today, I noticed that if I killed the stub, it often kept reappearing, each time using a different amount of memory.
I suspect it's CompanionLink for DejaOffice, the only program I know of that access my Outlook data; I just tried closing it from task manager, and the stub has not come back. But you mentioned other programs might cause this--which ones?