Permit only real identities, and have a verification process.

Have user terms that prohibit unlawful speech plus an indemnity (of little value but worthwhile).

Have a specialist lawyer review the recent cases on "mere conduit" immunity and take a view. If the view is for moderation then there is a greater chance of being liable for content that is allowed to remain.

Have all those accessing the site verify identity and tick box to confirm user terms and thereby waive all rights against the charity/forum/operators.

Log not merely user IP addresses but also MAC addresses and block all spoofed versions of either. Take special verification measures where apparently different users use the same IP address and/or MAC address.