I asked a question about a particular wifi adaptor performance. It looks like this wifi card is not well supported and probably never will be. How should I close/answer my question to let people know that there isn't a solution?
If you can provide detailed information, or information with a reliable source, to indicate that the card is unlikely to work, I'd say you should post your own answer containing that information. That answers the question, and will help others with similar problems to avoid wasting their time (or at least to know what the current state of support is, before attempting to get the device to work).
We get all sorts of questions about how to get something to work in Ubuntu, and sometimes the answer is that it just doesn't work (so far). Those are perfectly good answers (when they're correct), so I think yours would be too.
If it's still an issue you can test and post you can maintain and something you want fixed, I'd consider a bounty first of all. We get a lot of questions and it doesn't take long for some things to slip off the front page and into obscurity. Putting a bounty on it means motivated people will see it for a week.
Failing the outcome of a bounty, or if it's not something you can maintain or are interested in, I'd just delete it (there should be a delete link under the question).