A force stop will close the app, but you generally don't want to do that unless there's an apparent problem with it. With most apps, they go dormant once in the background. Basically memory on our phones requires the same amount of battery power whether it's used or not. It uses less power to recall a dormant app from memory than to initialize it from scratch, hence the "free memory is wasted memory" phrase. Android already manages this well without any help, and memory managers or trying to beat the system yourself can actually hurt battery life.
There are exceptions where apps still work as intended in the background and occasionally use some power. I.e. mail, messenger, weather, and other apps designed to sync every so often or give you notices. Those would eat up a little battery power, but force closing them should be a last resort. First check to see if there's a way to limit background usage in the app's settings or limit/stop sync activities. By force closing, you're not changing the app's behavior and the OS could initialize it again (because it's designed to keep memory full) without you knowing and it'd go back to doing its thing again. If you absolutely have to force close an app, you may as well disable it too, which freezes the app so it will never start again until you re-enable it from the app settings screen.