When removing scenery, the trick is to back out the way you came in.
Start FS, go to Settings>Scenery Library and uncheck the scenery to deactivate it, then click the delete button.
That will remove the scenery call from the scenery.cfg. So long as the call for the scenery is in the scenery.cfg, FS will go looking for it.
That's where those error messages come from.
Close FS.
If the scenery came with its own installer, there should be an uninstaller. Go to Start>Programs and it will be in the folder for that scenery. Run the uninstaller, and you're done.
If the developer did not provide an uninstaller, go to Start>Control Panel then to wherever your version of Windows lets you remove software.
In Win7, it is in Programs and Features. In older versions it was Add/Remove Programs or Add/Remove Software. Uninstall the scenery from there.
If there was no installer, open whatever you use to see the files on your computer (I use Windows Explorer). Find the folder for the scenery you want to remove, and delete it.
TIP: When you manually install a scenery, make sure the readme/installation instructions finish up in the main folder for that scenery. There will often be an AFCAD which is placed in Scenery\World\Scenery, and you need to know about that to avoid clashes with something you install later.
Hope this helps
MikeW