How to disable spaces?
If you have multiple spaces, you can delete them by exposing the Spaces (by default, Control-Up arrow), and then hover over one of the spaces with your mouse pointer. After a moment, you should see a round (X) appear in the upper left corner. Click and it will delete the space, repeat until you have only one left.

If you find that when you launch an app a space comes back, right-click on the App's icon in the dock, and look under the "Options >" menu. There may be a checkmark beside "This desktop." If so, selecting "None" and then closing the Space like before should stop that.

Edited based on comments: Mission Control does need to be re-enabled first. In the terminal, type

defaults delete com.apple.dock mcx-expose-disabled


and then

killall Dock


And wait for the dock to reload. Then, clear the unwanted spaces (or all of them!) and re-disable Mission Control.