where to refund kitaca

my itinerary is from Sapporo to Hakodate and then to Tokyo, where is the last place that i can refund kitaca, is it Sapporo or Hakodate or Tokyo?
thank you in advance for your replies.

Sapporo.

Refunds can only be done at the JR station within the card valid area of Kitaca.

For information on refunds, please click here. (Use Google translate)
http://www.jrhokkaido.co.jp/faq/qa0086.html
Here is the list of stations that can be refunded.
https://i.imgur.com/3yFiuPK.png

ticket office/counter : みどりの窓口 midori no madoguchi
refund : 払い戻し/払戻 harai modoshi
station name : 駅名 eki mei
Sapporo : 札幌
west exit : 西口 nishi guchi
east exit : 東口 higashi guchi
fare adjustment counter(= ticket gate) : 精算窓口 seisan madoguchi

However, if you have plans to come to Japan again within 10 years, you do not need to refund.
The expiration date of the card is "10 years" from the date of last use. (NOT 10 months)
Also, Kitaca can be used in the same way in Tokyo.
You can use Suica and PASMO cards trains and buses, and you can charge.
Of course, it can be used as e-money such as shopping(conveni and more), eating and drinking.
http://www.jreast.co.jp/e/pass/suica.html
https://www.pasmo.co.jp/en/use/
It is not only possible to return the card and reissue it by lost and damaged at other IC card area.
(Did not you use these cards in your past Japan travel?)

Can I use kitaca in Tokyo konbini? Thank you

Yes, and more generally you can use it in all stores that display the Suica or Pasmo logos.