Whenever I'm done with an application, I always exit using the back button. Somehow applications like Viber, WhatsApp, Facebook, Videos, Email, Google Maps, etc keep restarting on their own.
I have tried:
...but they still restart. Any idea how to solve this problem?
Let me first say: Do not use task killers. You are only making the problem worse. Why? Let me explain how the Android OS works.
Unlike normal OSes, Android does not actually let you "close" an application. You can leave the application, but that doesn't mean it has "closed" or "killed" it. This is because of multi-tasking: when you leave an application, you can come back to it as you were previously.
Instead, the Android OS handles the "killing" or "closing" of applications for you. It keeps track of a) how much free memory there is and b) how long it has been since you've opened that app. When memory is running low, Android will garbage collect and kill any unused applications. Android's OS task killer is very smart and can handle these situations far better than an Android Task Killer app that just kills apps left and right without any "logic" behind it.
Now, when you leave an application via back button or home, this doesn't mean the app is necessarily still running, just that it remains in memory. The only time an app will run is a) it is in the foreground or b) it has a background service.
In your case, it sounds like you are trying to kill background services. When you do so, the app will just go "oh hey, my background service is running any more, let's start it back up!" And you will then be playing a game of whack-a-mole. You'll be wasting resources and battery life because applications will be re-starting themselves. And if an application was writing to disk (your SD card) at the time you killed it, then you could end up with corrupted files and other issues. Rare, but it could happen.
The only need for a task killer is to stop an erroneous application in one-time situations. That's what force stop is for. You shouldn't need to force stop every X min. ONY when the app is misbehaving in rare occasions.
If you don't want all those apps running background services, check to see if they have settings to disable them. Look for things such as disabling background notifications or other "polling" for information/updates. Otherwise, uninstall the app.
You can find even more info over here in this thread and this thread.
Some of the applications (e.g - facebook, maps) are like services.. they keep restarting always. If other applications like Browser, Messaging are exiting fine then there is nothing wrong. It's just that some applications always want to run in background. Best option is to uninstall any of the applications which you don't use. If there is an application which you require, better let them run. Using task killers is not a good idea.
