As the site owner you can always access your data. In the web services panel you can make a complete backup of the database.
Go to http://(your site here)/config and log in. Use the lost password link if you've lost it.
Once you log in, in the top group of links - left side - select "create/restore backups" and on the resulting page you'll see that you have options to backup all or part of your data. If you're looking to migrate to a different forum system you'll need the database which is the bottom check box, and if you have any attachments you'll need the web files, which is the top check box.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .