Generally speaking, there isn't a right to force others to delete stuff you've posted to the internet - only stuff related to PII. If you anonymize posts (say, change the username to AnonymousCoward and delete real names, reset passwords, etc), you're 99% of the way there - and if the person in question comes back and says "you've not deleted PII that I posted in one of my posts", you can go and delete that or edit it out, you're not about to get fined if you make a best-effort attempt as a small company - only if you're obviously negligent.

There is no right to delete stuff that is not PII.

GDPR doesn't include a blanket right to delete your data.

And forums already had to protect against eg people under 13 registering, or people under 18 sharing nudes. Both of those pose significant risk to online services, but people cope.

Your comment made me wonder how services like archive.org that automatically save forum posts that may contain personal information are going to deal with GDPR.