Disable putting apps into the background?

Yes, if you don't want to see them in the recently used apps the only way to avoid that is not to use them (then they are not recently used) or don't look at the recently used apps.

Switch apps on your iPhone, iPad, or iPod touch - Apple Support

Very few apps run in the background and you turn them off if you want. The apps you see when you double-click your home button are recently used apps, they do not slow your phone down or eat up a lot of RAM etc. If you don't want recently used apps then don't use any of them.

I think there may be some misunderstanding here. Can you explain what you mean when you say you don't want apps in the background?

With a small number of exceptions (mostly music and navigation apps), unless you're using the app, it's suspended. It's not doing anything unless you have background app refresh enabled for that app or unless you have it set to give you notifications. Any memory that it might have been using will be released if it's needed.