Nobody prior to your post said anything about a "best" way, that was not the OPs question and neither of us said it was the best way.
The mods may be busy but they can handle it without too much effort and taking too much time away from your imaginary scenario. After all, it only takes about 10 seconds using the tools provided within Xenforo software.
While I agree that becoming a subscriber is important to the forum, spending $25 to delete their own post may not be appealing to a New Member.