Are you talking about the fact that the workspace automatically switches to a 3D workspace? As I recall, it prompted me the first time it wanted to do that. I said "No", don't switch. Are you sure you didn't answer yes, then from then on it's doing it automatically?
I believe you can get it back to where it doesn't (I don't allow it to switch mine)... Go into your Preferences - General panel and press the [ Reset All Warning Dialogs ] button. Then invoke 3D and when it pops up select "No" and check Don't prompt again (this is from memory).
Regarding undoing creating a 3D layer... Control-Z to Undo. It should go right back to the previous step.
-Noel