How to prevent apps running in background
Though there are apps around that would answer to your request (so-called "Task-killers" and "Memory boosters"), their usage is not to recommend -- simply because it would not help any (see Is it advisable to run a task killer app on Android). If those background processes are designed as services, they would simply restart - as would many other apps as well. Usually Android handles RAM very well, and those background processes should not use significant CPU power -- so on a decent phone this should not be a problem. However, on low-end and small devices with just minimal RAM and CPU, this can become an issue.

So what are your choices then? First, you could see to uninstall all those apps running background tasks -- provided you do not really need them. And, of course, if your device is not rooted, this cannot be done for pre-installed apps; but with ICS and higher you can at least "freeze" them.

If your device is rooted, your choices increase dramatically: you can remove (or freeze) those apps e.g. using Titanium Backup. You can use managers like AutoRun Manager to modify intent-listeners of apps (to prevent the start of certain event-based background processes, including those started on boot_completed). And you can configure thresholds for how your RAM will be used (i.e. when Androids internal Out-of-Memory-Killer should clean up your RAM) using apps like AutoKiller Memory Optimizer (don't mistake that name with a task-killer, it is not) or Auto Memory Manager.

Of course, you can also ask the developer to make those background services configurable -- and to leave it to you whether certain of them should be running at all. There's always the chance the developer sees reason and implements such a feature.