Refusing gifts - impolite?

I'm staying in Japan for 6 months, and I've been doing homestays in many different families. They always treat me very well, and in addition, often offer to buy me something as a gift.

I always feel I should refuse the gift, because I'm treated so well already and I don't want to be any more of a burden. At the same time I feel very rude, because the situation can feel a bit embarrassing if they insist to buy me something and I politely try to refuse over and over.

Usually they just state it matter of factly, "Oh, you like that? Well, it will be a gift from us", without asking, so I finally accepted and let one family buy me a gift. Of course I thanked them many times (and am planning to send them a package when I return to my country).

The question: which is the best course of action to not be impolite?

Although it depends on how the presenter show enthusiasm to press the gift on you...

If it's so expensive or really bad for you,
or already you received another gifts,
refusing firmly is not impolite.
But it's cheap and not cause of inconvenience, you maight want to accept it.

To send them something in return later is good manners.