You can logout from the application before you put it offline, which is probably best because you will also want to get rid of your data.
Anyone getting the old computer will have all kinds of interesting stuff they can steal, not just your Dropbox/Copy account.
When getting rid of an old computer, you are advised to delete all your files. Some programs will wipe your files and shred them as well, but of course to be really secure you will want to use a hammer and break the HDD into little pieces.
A whole range of answers are here:
https://askubuntu.com/questions/5757...-secure-manner
It includes command-line methods of deleting files, wiping previously-deleted files, instructions on demolishing the HDD
(acid baths FTW
) and what is probably the simplest and easiest way:
bleachbit, a handy program for cleaning up all sorts of things, which I mostly use to free up space by deleting translations for languages I will never use.
