It is impossible to turn off activities. You are already using activities if you are using KDE Plasma. Your desktop, or virtual desktops, are an activity. By adding a new activity, you are creating another group of virtual desktops, just like the one you are now using. The global settings, like how many desktops there are in a group, carry over from one group to another. The technical details of what is allowed differs from distro to distro and, in my experience, Arch and Chakra have worked the best. openSUSE had all of the settings, but was somewhat unstable upon changing them. Some distros allow a different wallpaper on each desktop, and some do not. I did not like activities to begin with, as my previous comment will show. I urge you to play with them, because they really are useful. Like I said, you are already using them and just don't know it, but they can be removed from the panel if you insist. Trust me though, it is worth playing with them for a while. When you get used to it, you will be zooming around in an uncluttered environment in no time.