You must not despise them just because they're poor.
Literal
Poor [because-だから] [called-という] only [by-だけで] them [object-を] despise [must-not-てはいけない].
~だからというだけで is an expanded version of ~からといって — literally 'just by the reason called being poor.' The という quotes the reason, and だけで limits it as insufficient justification. てはいけない is a stronger prohibition than するな.