I deleted Dropbox!!! heres how it worked for me

Goto "GO" at top of mac window. GO> Find Folder> type in ~/.dropbox (squiggly line + backslash + dot + Dropbox) then drag to trash. Once again goto GO> Find Folder> type in /library (backslash + Library) and delete Dropbox there as well.

I stopped the "extension being used" error by going to Applications> Utilities> Activity Monitor> click on DROPBOX in the list then hit the "X" top left. I had to force quit it, I was the FINALLY able to TRASH the app from Applications folder.

I am running macOSHigh Sierra