you can destroy them or use their settings to set them disabled.
If it is a bullet I destroy it and create when needed. For buttons and other objects I use disabled and enable as needed. I set all my buttons on the layout and make them visible and enabled as needed.
Yeah, I know that there are some objects which can be disabled via the settings.. I do that already, my problem was specific to bullets, particles etc. which have to exist at the start but can't be disabled fully (well, you could like setting the speed to zero etc. etc. but it's not THE solution for me either).