If you want to turn off an Applet, but you can't find what IFTTT account it's connected to, you can stop it from running by removing access to IFTTT from within your service's account.
Here's how to do it:
Log into your account for that service (ex: Twitter), find the section for connected apps in your account settings, and remove access to IFTTT. This will disconnect your service from IFTTT and stop all Applets from running.
Here are the settings links for common services that people ask for help with:
If you do want other Applets to run on that service, you will need to reconnect IFTTT to that service and turn on the specific Applets you want to use.