Answer by cypher for たたく versus ぶつ in a JLPT question

• 尻をぶつ:

  Means to spank someone as punishment for something which has been done (in the first case because of naughtiness.)

• 尻をたたく:

  Means to give someone a good hiding without those connotations (in that case because of laziness.)

• 打つ:

  Doesn't work because ぶつ is used for people and 打つ for inanimate things.

• 殴る:

  Doesn't work because it's more for violently hitting/continuous beating of someone with a stick/fist etc.

なまけ者 means "lazy person" and なかなか仕事をしない means "the (younger brother) stays lazy" in this context I believe, so the sentence would translate to "unless the lazy younger brother is given a good hiding, he stays lazy."