Enabling P2P Fundraising allows your supporter's to raise support on behalf of your organization. To turn off general P2P Fundraising, go to the P2P Tab under the Donor Portal section of your Dashboard.
You can also remove P2P Fundraising on specific campaigns from the Edit Campaign page. Just uncheck the "Allow P2P Fundraising" box.
To learn more about P2P Fundraising, check out these articles: