After installing various applications or services on your system, the computer may start these up at boot or log-in. OS X supports a number of different ways to automatically launch applications and services, some of which are a little more obscure than others and may result in applications launching even though they are not in obvious start-up items locations.
Keep in mind that the approaches I mention here will not remove the program from your computer, but instead will just prevent it from being launched.
Some common methods to launch applications are the following:
Log-in Items tab
Account-based log-in items are the most common way items are automatically launched, and opens them either directly (i.e., folders and applications) or through their handling program (i.e., for documents) when the user logs in. There are several ways to add an item to the log-in items, but the easiest way to manage these items is to go to the "Accounts" system preferences and select the "Log-in Items" tab.
In the list that displays, you should see a number of items, hopefully including the program that constantly launches when you boot the system.
In OS X, the "Startup Items" folder is located in the global users directory, and can be used to open applications or services at boot, instead of only when a user logs in. Generally these are helper services that run as "root" for various applications, such as the "Parallels Transporter" utility that ships with Parallels Desktop.
These items are stored in the /Macintosh HD/Library/StartupItems/ folder, and are usually placed there by application installations that need them, but can be put there manually as well. Go to this folder to see if any files or applications in there that are associated with the application that launches, and if so, try removing them.
Launch Daemons and Agents
Launch Agents and Launch Daemons are the way Apple manages the dynamic and scheduled launch of applications and services in OS X. The launch agent and daemon files are simple property lists that interface with the launchd process (the parent process for root and user processes), and provide parameters and conditions for the launch of various applications. Sometimes system tools such as firewalls will be launched through these means.
To remove a launch agent or daemon, you can go to the /Library/LaunchAgents/ or /Library/LaunchDaemons/ folder and locate the property list associated with the application that starts up automatically, remove it, and then restart the system. The property list can be unloaded from launchd using command-line tools, but if the property list is missing at start-up, the system will just bypass it when it loads.
Though Apple primarily uses and advocates the use of launchd to manage scheduled processes, the old method for doing this on Unix systems is still available in the system, and may be used by various programs on installation to run tasks at specific times. This process is called "crontab," and can be managed through the command line.
To see if a cron entry is being used, go to the Terminal and enter "crontab -l," which will list all available cron entries. If none exist, the system will output a line that states so, and you can use other approaches to troubleshoot your application launches. If it does list an entry that is relevant to your application, then you can delete it by running "crontab -r" in the terminal to remove cron entries.
There are other means for launching applications as well, which include scheduler applications or those that support the launch of scripts in alerts. These include calendaring programs such as iCal, so be sure to check your calendars for any alarms you may have linked to opening various applications.
Additionally, various third-party utilities that you may have installed could be launching other applications. These utilities can include backup programs, maintenance utilities, and also simple menu extras. Take a look at the settings for any application you have installed and that you run continuously, to see if it is responsible for launching other programs.