The majority of English language instructors in Japan
aren't English teachers. The big schools like Nova hire people straight out of university with
any degree (and even then the degree is only to satisfy immigration). The sorts of wages offered should give you a clue. Most adverts for language instuctors in Japan offer a monthly salary of between 250k and 300k. A newly qualified English teacher in the UK will recieve 400k. It's only a year extra training on top of your degree, paid for by the government, then a year of working in a school before you're qualified. Suffice to say, most people who are serious about teaching will do this instead.
I would also be wary about "CELTA = serious about teaching". Again, I can only talk about my own country, but a lot of people will do this course as part of their degree course (for free)
precisely because they want to spend a year travelling after university.
Personally, I'd say the best way to filter out the travellers and people who aren't serious would be to specify a minimum of 2 years of experience teaching
in Japan (not only that, but you'll end up with applicants who are a bit more stable and established in the country). I would specify a CELTA or equivalent in the advert, but don't reject applicants who don't have those qualifications (firstly, you might get some of the more confident non-qualified applicants, and if you do find someone who's suitable that doesn't have the qualifications, you can offer them the job on a slightly lower wage than advertised

). And be careful at the moment, because Nova employees are deserting like rats from a sinking ship, so be careful not to employ someone who's just taking a job so they won't have to go home early.