How can I prevent apps from running in the background? Possible Duplicate:
How can I stop applications and services from running?


There is already a similar question to this one, but mine is narrower in scope and not addressed in the other question:
Consider the case of an app that meets the following criteria:

Let's take a file manager as an example, but it could be a different app just as easily. The specific app isn't important to this question.
This app has no legitimate need to run in the background. I only need file management when I'm actively using the app. (In case my specific example does turn out to have a need, suppose for the sake of the discussion that it actually doesn't.)
The fact that it runs in the background is confirmed by a system monitor.
Even though I believe that it has no legitimate need to run in the background and don't want it to do so, I don't want to uninstall it.

Can I prevent such an app from running in the background?
Practical application
To illustrate the need for this specific question, and why I'm not naming specific apps, consider what I had in mind when I wrote this question:
I would like to go through my apps and decide which ones I want to run as services. Some should run without restrictions. Some should never run. Some should only run when some set of conditions is met. Using the information from an answer to this question, I could implement this--possibly by some custom scripting, possibly using Tasker, or possibly using another approach. Trying to handle this via each individual app's settings would be well-nigh impossible.
Of course, if what I'm asking in this question isn't possible, then this application of it isn't viable.
Notes based on the comments and answers

The reason why I want to do this is irrelevant. Perhaps it's to save battery. Perhaps it's another reason. This question is about how, not why. A warning that a particular solution could have negative side-effects is useful if presented along with that solution. Saying, &quot;It's impossible&quot; is legitimate if true. Saying &quot;Don't do it&quot; isn't useful.
This is not a duplicate of the question I linked to in my first paragraph. That question was about saving memory. This one isn't. That one said, basically, &quot;Don't try.&quot; That's not a valid answer to this question.
The specific apps are unimportant, which is why I was vague. I did give a hint as to a specific example I had in mind, but I'm looking for a general answer here.
A solution must also prevent an app from automatically restarting; otherwise, it isn't much of a solution.
A number of people have stated that this is a duplicate. Let me try to explain more clearly why it isn't:

This question is about how to prevent an app from running in the background, which includes stopping it from restarting if killed.
The answers so far have fallen into a few general categories:

You shouldn't try, because doing so is unnecessary. I'm not disputing this point at this time, but &quot;You shouldn't try&quot; is very different from &quot;You can't.&quot; Logically, it's addressing a different question, namely &quot;Should I try to kill apps that I don't want to run?&quot;
Use method X to kill the app. Unfortunately, when I tried said method, it didn't prevent the app from restarting, just like others predicted. So, it isn't really an answer.
Ping the developer/it's a bug. This is the most useful of the suggested solutions, but my question was looking for a generic solution.


The other questions that this question has been said to be a duplicate of cover the same ground as in my previous point. Thus, my specific question has not been addressed, despite many claims to the contrary. It's appearing, however, that the answer is a simple &quot;You can't&quot;.
There's a related question on meta. In Jeff Atwood's answer, he says in part, &quot;It's generally sufficient to link to the other question and explain why it doesn't meet your needs.&quot; The whole thread is apropos to this question. I've done this since the beginning, and provided further details in subsequent edits.
tl;dr
It's ok if the app in background is just an activity. Activities just consume some memory are usually  not able to run computations in the background. Therefore no CPU or battery is used for them. But if the app in background is running a service:
Ping the developer
Ping the developer! One of the greatest daemons that come with Android are Services that run in background for no reason or when a simply interval check with Android's AlarmManager would be sufficient. We need to kill them all with fire and not by ignoring them. Everything that does not involve a fix by the developer is just a dirty hack that by experience will result in other negative side-effects.
Activities that run in Background
Pure Android apps and their activities usually stay around in memory for a while after they were put out of view. But at some point they may get deleted by Android's ActivityManager if there is an ongoing memory pressure. This is intended behavior, we (the users) want it that way and it works good in recent Android versions. Therefore there is nothing wrong if you see your file manager hanging around in memory.
Services that run in background
But, if it's a service that you see running forever without apparent reason (and without the possibility to disable the service somewhere in the Apps settings), you should contact the developer. In 99% of all cases, there is no reason to waste your precious memory by doing so. Android provides with intents and the already mentioned AlarmManager enough facilities to avoid long running services. Sometimes developers will response with &quot;but Google Maps does that too&quot;. I argue that this is never an excuse to point at others, plus it doesn't provide a valid reason to run a background service.