You shouldn't despise someone just because they're poor.
Literal
Poor [just-because-からといって] person [object-を] despise [should-not-すべきではない] [emphasis-よ].
べきではない ('should not') is a softer prohibition than てはいけない — it expresses what one ought not to do rather than an absolute rule. The sentence-final よ adds gentle emphasis, as if giving advice to a friend.