Learn Spanish (Latin America): Se prohíbe obstruir el pasillo del autobús. - It is prohibited to obstruct the bus aisle.

In this sentence, we are using the reflexive passive (pasiva refleja). The grammatical subject is actually the infinitive phrase obstruir el pasillo del autobús (obstructing the bus aisle). Because infinitive verbs are always treated as singular nouns when they act as subjects, the verb must be the singular prohíbe.

If the sentence was about obstructing the aisles (plural), would the verb change to the plural prohíben?

No, it would still be Se prohíbe obstruir los pasillos. The verb prohíbe agrees with the infinitive obstruir (to obstruct), not the word pasillos. As long as the main subject is an infinitive, the passive verb stays singular.

Is this an impersonal se or a passive se sentence?

It is a passive se (pasiva refleja). While it translates well to the impersonal English concept of it is prohibited, there is actually a defined grammatical subject here: the action of obstructing (obstruir). If the subject were a plural noun instead of a verb, the verb would change to plural (Se prohíben los perros). True impersonal se sentences never change to plural.

Could I change the word order and put the action first, before se prohíbe?