It depends on how the app decided to do it.
If it starts up when the computer boots, look in /Library/LaunchAgents or /Library/LaunchDaemons. They might also use /Library/StartupItems which is deprecated.
If it starts on Login, look in those same places inside your Home folder.
For the first two, it will be a .plist file named in reverse domain name format.
For startupItems, it will be a folder named somehow reflective of the software.
If they are in one of the Launch folders, they are generally controlled by launchctl. You can “unload” them so that they do not run.
The basic syntax is:
sudo launchctl unload -w /Library/LaunchDaemons/com.company.software.plist
The -w options tells it to write the bit to the file to keep it unloaded. If you wish to run it again and have it always run:
sudo launchctl load -w /Library/LaunchDaemons/com.company.software.plist
See here: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/ man1/launchctl.1.html
If you want to get rid of it totally, the software should have an uninstaller. Use that.
If it doesn’t, you can just unload it, then delete the plist.