You must not look down on people just because they're poor.
Literal
Poor [because-だからだ] [saying-といって] people [object-を] despise [must-not-てはいけない].
~だからだといって is an expanded variant of ~だからといって — the extra だ reinforces the copula before the quotative という. The core meaning is 'just because someone is (poor).' てはいけない marks the prohibition.