I can't delete dropbox, nor kill it in activity monitor

Every time I turn on my computer, an annoying alert box asks me to log in to Dropbox. However, when I enter username and password it tells me that my OS is too old (I have Sierra 10.12). So I can't log in. I tried to delete it, but Finder says there are extensions in use. In fact, if I open Activity Monitor I see Dropbox, but is grey, as disabled, I cannot click on it and the PID does not appear (so i can't kill it from the terminal). In short, I can neither log in nor delete Dropbox! Some idea? Thank you!