A person who becomes a member of the forum, makes one post, and is never seen or heard of again.
Some ask for advice while others are would-be spammers. I wouldn't condemn the originator of this thread just yet, but dropping in the name of a product does make one wary. Another recent one, whose written English was similar to that of someone from south-east Asia, joined at 03:30 GMT; there's no harm in that if they want to discuss their walking experiences, but more likely there's a different motive.
I am guessing the term is derived from "one hit wonder" which is used to describe a pop star who only ever makes one successful recording.