It's sad I have to even say this. Believe it or not I have already tried finding an answer to this question.
How do you close a workspace in GNOME(specifically, Ubuntu 18.04)?
Workspaces are automatically closed when you remove all the windows from it and switch to another one. I'm not entirely sure, but I think on Ubuntu 18.04 it might also keep one open at the bottom so you can drag windows into it.
Also see: https://askubuntu.com/questions/1047051/remove-a-gnome-workspace