Are you sure you mean "Unsubscribe". This applies when someone is receiving emails from your forum. You cancel emails by going to:
Options > Your subscription: [forumname] (edit)
On the screen that appears you click the "Unsubscribe" button. This leaves all your posts intact and just stops you getting the emails.
I suspect you mean that someone deleted their account on your forum. To do that you go to:
[yourusername] > account settings > Remove your account
There you get a screen that says:
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Remove Your Account
Do you really want to remove your account and all your posts from Nabble Support?
[Yes, please remove my account] or Cancel
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Normally a user will just lose interest in a forum and stop visiting. Usually, people only delete their account if they have been offended in some way by the owner or others posting on the forum. There is no way to stop them choosing to delete their account.
If it really was a user error, they will have received a copy of all posts. You can ask them to submit the file they received and Nabble will be able to recover the posts from the file if submitted to them.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.