Spammers edit old comments to add spam links
Summary
There is an ongoing problem in the forum where spammers edit their comments some time after posting to insert spam links.
How it works
A new account will post an innocuous comment. They'll often say something generic like "I have the same problem, hope someone can help" or "This post helped me, thank you!" These comments get approved, as there's nothing objectionable, and legit users do post comments like these occasionally. Even if we're suspicous the comments come from a bot, there's no real justification to deny them.
After posting, some time will pass. This could be anything from a day to a couple of weeks. The thread has usually become inactive since the spammer posted, and no-one's paying attention to it.
The spammer will then return and edit their comment to include a spammy link. These edits are only noticed if someone else posts in the thread later, and that frequently doesn't happen.
Suggested solution
I would suggest that comments are locked after a certain period of time. I'm open to opinions as to how long, but I'm tentatively thinking 12 hours?
Many people edit comments soon after posting, perhaps because they made a typo, or they think of some additional details they'd like to include. 12 hours is more than enough time to fix any small issues with the initial comment.
If they want to give more details after their post is locked, they can simply add another comment. There's no need to summarise everything in the initial comment (the way users do on Reddit, for example), as there are no nested comments, and everything is posted in chronological order, so it's easy to follow the discussion. I don't see any real downsides to locking comments after a number of hours, but it should help to cut the spam.
Moderator powers
I would suggest that forum moderators retain the ability to edit comments without any time limits, in case anything slips through.