Alternatives to the Ablative of Agent

There are two: per + accusative and operā + genitive.

Recall that operā (sans ā/ab) is operating as an Ablative of Instrument
These are constructions where the agent/instrument sense breaks down: with per, animate agents seems to work as instruments, and with operā, the (instrumental) labor of an agent take the place of that agent.

Agents with Per Construction

Caesar was informed by scouts: Caesar ab explōrātōribus certiorātus est.
Cesar was informed by scouts: Caesar per explōrātōrēs certiorātus est.

He was trampled by charioteers: ab origīs compressus est.
He was trampled by charioteers: per origās compressus est.

Agents with Operā Construction

Caesar was informed by scouts: Caesar ab explōrātōribus certiorātus est.
Caesar was informed by the work of scouts: Caesar operā explōrātōrum certiorātus est.

The walls were cleaned by a slave: mūrī ab famulō lōtī sunt.
The walls were cleaned by slave’s labor: mūrī operā famulī lōtī sunt.

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While we’re at it, note the odd forth principle part of wash: lāvō, lāvāre, lāvī, lōtum.

Really, it varies: it can also be lautum or lāvātum, depending on the author.

The Essential AG: 405b