One of the best ways of doing this is to rework old discussions (cf WhatIsReworking). If you see an old fossilised argument, delete everything that no longer has value (if indeed it ever had value), and leave a summary of what's been learnt.
However, you can't force forgiveness on the community against its will. If you've been heavily criticised on some page, leave it to someone else to decide that it's no longer relevant (though you may want to prod them a bit).
This applies even to CommunityExiles? - the community must learn to forgive and forget their crimes.
The above text is PrimarilyPublicDomain
See also: RememberThePast. Remembering the details of the past allows other people to draw their own conclusions from the old fossilised argument. It is important to find closure, however. Reading the old fossilised argument should not revive the conflict; RememberThePast focuses on the analysis (historical perspective) of past conflict.
See also: InterpretationRequiresContext?. If you do read the old fossilised argument, don't just stop there. Read on and determine whether closure was found at the time. What led up to the conflict, what kind of resolution was found -- all of this context is of prime importance to understand and evaluate the past.
ForgiveAndForgetInSoftware attempts to remove traces of past conflict; what is really required by the people involved in conflicts, is more. Forgiving is just the first step: