If it is not in the regular user Login list, then it is most likely in LaunchDaemons.
There are two locations for these, one is for Apple daemons only: /System/Library/LaunchDaemons
The other is to include 3rd party processes:
/Library/LaunchDaemons
You may also need to remove from:
/Library/LaunchAgents
Simply remove or zip the process you don't wish to auto-launch & it should be gone at next boot.