You can, but...
There is nothing forcing you to accept any particular answer. You may accept any answer you want, as Rand mentioned in his answer -- but also as he mentioned, accepting a "wrong" answer may earn your puzzle downvotes. Acceptances mark answers with a sort of "official" status, and if that seal of approval is given to a bad answer, it reflects poorly on the question as well.
... it goes against the purpose of the site.
While you can accept any answer you want, you should accept the answer that best explains the solution to your puzzle. What is "best" is often subjective, of course, but just accepting an answer based on upvotes alone will often not lead to that. Things like the Hot Network Questions sidebar often attract people who are not Puzzling regulars. When that happens, answers that are short (or even jokes, not intended as a serious answer at all) are disproportionately upvoted.
And it suggests more fundamental issues with the puzzle.
The fact that you can accept answers based on upvotes suggests that this is more of a game than a puzzle. As quoted in the "open-ended questions" meta post, Rubio also expressed the problem:
The danger - and I think we're correct to avoid it, and it's something Deus alluded to in an earlier comment - is that instead of a puzzle what we really have is a game: find the best thing you can come up with. (I've even seen some that are: best solution in __ days is the winner)
Based on the mentions of "points for style (and perhaps humor)", it sounds like this is applicable here. From your description, it appears that the goal here is not to set up a puzzle with a definitive solution, but a question where many people submit answers, and then one is given the checkmark after an arbitrary amount of time.
Also, given that this is apparently a puzzle based on the English language, it may run into an additional issue of answers being subjectively correct. What counts as an English word, or grammatically correct sentence, is very difficult to define. If this isn't precisely defined, whether an answer works is subjective. And subjectivity of answers is another thing that makes questions off-topic.