When one of these forces (Ruin or Preservation) is defeated, a human dies with it. But the force keeps going. Is the human an incarnation of the force, or is the human using the force?
The human is inhabiting the force. In the epigraphs of The Hero of Ages, there is some discussion of how a god's body is its power, and how gods occupy power in the same manner in which humans occupy physical bodies.
The body of a god—or, rather, the power of a god, since the two are really the same thing. Ruin and Preservation inhabited power and energy in the same way a person inhabits flesh and blood.
The Hero of Ages
The human bodies that you see are those of Leras and Ati, who became Ruin and Preservation, respectively. Effectively, the Shards had become their bodies, even though the originals still existed in a sort of scattered, potential state (and thus reappeared when they were killed). Expanding upon this with information from other series in the same setting, the Cosmere, we see that when Ati-as-Ruin is killed, his spirit appears in the Cognitive Realm and rapidly passes on to whatever fate awaits most of the spirits of the dead, which (according to the author) is intentionally left vague. Without his power, his body, Ati is little more than another dead human. By contrast, Kelsier, a human who died when his physical body was destroyed, is able to latch on to enough power to stay alive (after a fashion) in the Cognitive Realm. This is further illustration of the power-as-body idea.
In fact, all of the humans that you see as Shards, whether Preservation (Leras, Vin), Ruin (Ati), or the other Shards scattered across the Cosmere, started out as relatively ordinary people who assumed a portion of that power, which effectively became their body (subsuming the original and effectively destroying it for most purposes) upon ascension. In this sense, the human is certainly not the incarnation of the force. Knowledgeable individuals in-universe refer to the humans who take up Shards as Vessels, but this is something of a misnomer: initially, the power flows into a human body, but the ultimate result is that a human mind takes over the power and uses it, effectively, as its new body.
As a side note, the Shards were once part of an entity or force called Adonalsium. Current evidence seems to point toward Adonalsium having been a thinking being that created the Cosmere, as suggested by one of the most knowledgeable academics of the Cosmere (Khriss), so in a sense one could say that Leras and Ati, who were present at the Shattering of this entity, are human minds inhabiting pieces of the corpse of God.
However, unlike a normal body, which is part of the Physical Realm, Shards such as Ruin and Preservation are part of the Spiritual Realm. Hitting one of them hard enough that it kills the consciousness controlling it is not enough to destroy the Shard. It is unclear that anything is, although they can be Splintered into smaller pieces.
As a side note, although Leras and Ati were not incarnations of the forces they had become, the matter is complicated by the fact that with its vast powers, an relatively unopposed Shard could create a sort of puppet body that it could manipulate, which would look much like an incarnation, as we see elsewhere in the Cosmere, while some evidence also suggests that a Shard can create a sort of avatar of itself with relative autonomy, although this capacity might be specific to the Shard of the same name.