A bubble would never survive the intensity of the thousand weaboos that try to sneak into the country every day.

I believe the only realistic solution here is to place the islands of Japan on a steel rocket ship, coat the ship in concrete, bury it on the moon, knock the moon out of orbit, send it to another galaxy, have aliens in this galaxy coat it in a jelly-like substance and carefully place the whole thing into a super-massive black hole that will send it to another universe.

I realise there are a few flaws with this plan but with help of all major world nations, we might be able to achieve it.