The reason you put this riddles online is to get to know people who love this kind of challenge. This is still the case if the solutions are available online, even though I would include a plead into my job ad not to post solutions online for fairness with respect to other potential applicants who mananged to solve the riddle.

When in a job interview, it should be very easy to find out whether the person found the solution online or he "cheated"; simple ask some detailed questions how the person came up with this and that part of the approach.

The person that knows how to Google for solutions may be the right person for the job.

Perhaps filtering out the people who can neither solve it nor Google it is enough of a filter by itself. Not suggesting it should bypass more than initial screening, naturally.