Within your Lending Club account, you're only able to pause automated investing. By pausing the automated investing, the account still has the feature enabled, but should make no further investments on your behalf. To pause the service, go to this page: https://www.lendingclub.com/portfolio/primeReport.action.
At the bottom of the page, check the box "Pause Automated Investing". See the image below for details.
If you wish to entirely cancel Lending Club's automated investing service, you must contact their Investor Services at firstname.lastname@example.org