Learn Portuguese (Portugal): Ninguém disparou contra a polícia.

In Portuguese, you only need a double negative if the negative word comes after the verb (like Não disparou ninguém). Because ninguém comes before the verb here, you drop the não.

Why does the sentence use contra for 'at'? Could I use a or em instead?

The verb disparar usually pairs with the preposition contra when you mean shooting 'at' a target. You can think of it literally as firing a weapon 'against' the police. Using a or em here would not sound natural.

Could I use the verb atirar instead of disparar here?

Yes, you could say Ninguém atirou contra a polícia. The verb atirar means both to throw and to shoot, while disparar specifically means to fire a weapon or trigger something. Both are very common in this context.

Why is it a polícia (feminine) instead of o polícia (masculine)?

The word polícia changes meaning based on its gender. As a feminine noun (a polícia), it refers to the police force or the institution as a whole. If you used the masculine (o polícia), it would mean one specific male police officer.