Poor person [object-を] fool [to-に] do [must-not-てはいけない].
馬鹿にする is the kanji-written form of ばかにする ('to make a fool of'). 馬鹿 literally means 'horse-deer' — a famous etymological curiosity said to derive from a Qin dynasty story. ~てはいけない expresses a firm prohibition.