The idiom não querer/poder ver [someone] nem pintado is a neat equivalent of the English idiom cannot bear the sight of [someone] (The FreeDictionary). It means I cannot stand that person; I dislike or even hate them.
Now, I am a native speaker and I’ve always used that idiom without giving much thought to the exact meaning of pintado. I’ve vaguely interpreted pintado as ’with their faces disguised with paint’. But now that you’ve asked, I looked around, and it looks as though the original idea was that I don’t even want to see a painting of them. Here’s the 1868 Francisco Solano Constâncio’s dictionary (my boldface):
PINTADO […] Não poder ver alguem nem pintado, ter-lhe grande aversão, não lhe querer ver nem a imagem [not even wanting to see their image]
I’ve also found an old variant: não querer/poder ver [someone] nem pintado numa parede, i.e. ’not even painted on a wall’, so again a painting, not paint on one’s face. Here’s from the 1832 Contramina (old spelling):
[…] não inquiete mais os portugueses, que o não soffrem, que o não querem ver nem pintado em huma parede [don't disturb the Portuguese even more, who do not suffer him, who do not want to see him not even painted on a wall]
And an even older example, from the 1712 Sermoens Varios do Padre Simam da Gama. Note the “as they say”, meaning this was an idiom already in those days:
[…] aquelle mesmo que affirma nos traz nas meninas dos olhos, nos naõ pòde ver dos olhos, nem ainda, como dizem, pintado em huma parede [he who avers that he has us in the pupils of his eyes [i.e. ’loves us’] can’t bear the sight of us, not even, as they say, of us painted on a wall]