You might be able to go to the Account Management page then scroll down to "My Products" click edit. There might be something along the lines of "Remove Google+"
EDIT: Perhaps you should have done a google search. I just did it and I found this link. Clearly written instructions on how to delete Google+.
Step 1: Click on Settings icon on the top right of the Google Plus screen when you are logged in and click on
Step 2: Click on the Account Overview Link in the left pane and the click on the link towards the bottom of this page which says
Delete profile and social features – Delete Profile and associated social features
Step 3: On next screen, select the option which says
Delete Google+ Content IMPORTANT. Then click the check box at the bottom of the page which says
I understand that deleting the service can’t be undone and data I delete can’t be restored.. Then click the button which says
Remove Selected Services. This is it.