I don't know what system you are using, but regardless, it can be done. I'm strictly an Android user, but here are simple instructions for other platforms. You can permanently delete your Google profile and then scroll down the page to remove selected services. You would NOT choose your Gmail account, thereby leaving it intact. I did this once and found constant annoying pop ups announcing to me that google+ was disabled. Some things (I can't remember exactly what they were at this point) were not able to be accessed because of this. I found that it was too annoying and wound up reinstalling it.
https://support.google.com/plus/answer/1044503?hl=en