Learn Spanish (Latin America): Se sancionan los daños al torniquete.

In the reflexive passive construction (pasiva refleja), se isn't the subject of the sentence. The grammatical subject is actually the thing receiving the action, which here is los daños (the damages). Because los daños is plural, the verb must also be the plural sancionan to agree with it.

Is it ever correct to use the singular and say Se sanciona los daños?

No, that is a common mistake. When you are talking about things rather than people, the verb must agree with the noun. The impersonal construction, which keeps the verb singular regardless of what follows, is generally only used when talking about specific people introduced by the personal a, like in Se sanciona a los pasajeros.

The English word sanction can mean either to approve or to penalize. Which one is sancionar?

In Spanish, sancionar is firmly on the penalize or punish side. While formal legal texts might occasionally use it to mean ratifying a law, in everyday Latin American Spanish, it almost always refers to handing out a penalty, fine, or punishment.

Why do we say al torniquete instead of del torniquete if the damages belong to the turnstile?