To disconnect a service from your IFTTT account, you first need to visit the individual service page. This can be found through your services, the explore page, or the services page when logged in to IFTTT.
When on the service page, select Settings, and click Remove.
Note that this will remove all Applets and connections for that service from your IFTTT account. If you choose to reconnect the service, you'll be able to restore any removed Applets from your IFTTT archive.