Turn off opening new Spaces/Desktops AT ALL

On a side note, the green button will maximize (full screen) the app in question in its own space. If you use OPTION+GreenButton, it will maximize the app to the current space. The same can be achieved by double clicking the app window itself.

For apps launching in their own space, maximized, use the green button to make them smaller again, and then close them (Menubar > AppName > Quit). MacOS should remember that last setting, and when launching the app again, it should open smaller.

You can turn off Mission Control using Terminal, however this is not the way I suggest people do it.

The command to DISABLE Mission Control (in ye olde days, called "Expose"), use the following command:

defaults write com.apple.dock mcx-expose-disabled -bool TRUE && killall Dock

If you wish to re-enable it, use:

defaults delete com.apple.dock mcx-expose-disabled && killall Dock

That said, I suggest downloading a tool called ONYX, by Titanium Software. It's free, and one of the settings it can manage for you is turning certain features in MacOS on or off, using the correct commands and syntaxes in the background, so you as end-user don't need to worry about it, and won't brick your system.