[FusionBB Versions: 3.0+]
Starting with Version 3.0, administrators have the ability to force all users to have to re-accept the forum rules before they can participate on the site.
This is useful if you make a change to the rules or simply need to remind everyone of your existing rules. Clearing the agreed date will cause every user to have to read and accept the rules again on their next visit back to the forum before they can continue.
To clear the agreed date, go to:
AdminCP >> Database >> Re-Sync Data >> Help Clear Rules Agreed on Date
Once you've clicked the link, the date will be cleared. (No confirmation is needed.)
(Thanks to FBB Developer Josh Pettit for this info.!)