Learn Portuguese (Portugal): Não atires o lixo para o chão, é proibido. - Don't throw the trash on the floor, it is forbidden.

Because this is a negative command (negative imperative) for tu. In Portuguese, negative commands don't use the normal present tense. Instead, they use a form called the present subjunctive. For -ar verbs like atirar, the tu ending swaps from -as to -es.

How would I say this formally to someone I address as você?

You would say Não atire o lixo. For você, negative commands also use the subjunctive, which means the ending for -ar verbs changes to -e.

Why use para (to) instead of no (on the) for "on the floor"?

The verb atirar (to throw) involves movement towards a destination. Portuguese uses para here to show the direction of that movement, whereas English focuses on the final location and says "on the floor".

Why use é (ser) instead of está (estar) for "is forbidden"?

We use é, from the verb ser, because the rule against littering is a permanent law or general fact, rather than a temporary state.