Add this to your Terms of Use. It protects you from pretty much anything any user request outside of the export function you can do from ACP. It basically says that anything posted on the forum is owned by the forum admin/moderators and the poster can not request the content to be deleted or exported. If users of your (or any site for that matter) ignores the terms you can never be held accountable for anything.
"Certain aspects of the Site may allow you to submit or post a variety of content to the Site, such as text, images, files, videos, and other content, either directly to the Service or through a Third Party Service (collectively, "User Content"). Please be aware that User Content is not confidential and may be accessible by other users and the public. Moreover, by submitting or posting User Content to the Service (either directly or through a Third Party Service) you grant this Site a royalty-free, perpetual, irrevocable, non-exclusive, sublicensable, assignable, unrestricted, worldwide license to use the User Content, together with all consents or waivers (if any) necessary to distribute, publicly perform, publicly display, transmit, communicate to the public and modify the User Content, by any means and in all media formats and channels now known or hereafter devised in perpetuity, and to advertise and promote such use, without further notice to, or permission from, you or any other person, and without compensation or reference to you or any other person. As soon as you decide to delete your User Content(s) or request a account deletion and at our discretion, we will irrevocably delete it completely from our server(s) within two weeks and will no longer make use of the above-mentioned license in the future. This excludes content (text, images, files, videos, and other content) that have become so essential that we have a legitimate interest in continuing to host them."