How do you completely get rid of Google Plus and still use Gmail and not have it pester you about registering for Google+?
I don't know about the prevention of pestering part (that's internal marketing, after all), but you can delete your Google+ profile.
At the bottom of your Google+ settings is a link:
On the subsequent page, you can either
I expect the first is what you want. The page also notes:
Which further links to this Google support page about "Downgrade from Google+", which also happens to give a step-by-step on how to delete your Google+ account.
(I didn't follow the steps all the way through, because I want to keep my G+ account.)