I found a nice movie on youtube.
This video's Japanese title is "ハロウィンの夜、魔女がスパンキングにやってくる"(lit. In Halloween night,a Witch will visit you to spank you).
So Chihiro is spanked by a witch(Ruka,her roommate).
Of course,Japanene halloween is NOT something like above...
cf. http://www.japantimes.co.jp/news/2014/10/31/national/halloween-japan-2014/
see also (official blog post in Japanene) ;
http://handsp.blog60.fc2.com/blog-entry-485.html