If an actor has flag bPushable = false, it still is being pushed by thrusting damage like melee weapons. It could be original behavior, yet I think it IS a bug.

No it's not. bPushable flag means that a thing can be pushed by touching it.

So, there's no decent way to prevent this thrust?

No really, you can prevent most of the thrust by having a big mass, that's how Hexen does this.