What somebody lefts deliberately to the public keeps public. This is a very basic rule people should educated in. And this is the basic rule that applies to public commentary as well.

Because this is the way the culture works and evolves: people produce a public statement and others will build upon that. If some person decides to retract their statement this thread will be destroyed as well as the source of references that were made afterwards.

(This means: a decision to withdraw a comment will affect the statements (and therefore the rights to express) of other people who "build" upon a comment.)

We should not start to mix it up with *ownership rights*. Think about books, music etc. - you cannot withdraw the idea. You even can not withdraw all copies from the market when it got published (even if some companies try it to place a "better" version). Of course u might try to purge the net - but there might exist always a "cache" and "archive" where some copird survive.

There might be some workarounds avoiding the problem:
1. As site admin check comments before they got published to the general public of your site. This way you can avoid legal annoyances and stupid statements that might backfire to the commenter.
2. Check that the date of the commentary is displayed correctly. No reasonable person will judge you about commentaries you did 20 years ago.
3. Think about mechanisms to ensure and display the realness of a "person". That doesn't mean the obligation to leave a "real name" - but connections to profiles that hold some information (for example a commentary record) might be helpful for that.