Actually what “actor->Destroy()” does is inside the function: “actor->GetWorld()->DestroyActor()”. So they effectively seem to do the same thing. I tried it and the result is same.
It seems to be about garbage collection. Forcing garbing collection does not seem the work in some cases as in my original post.
I think I will solve my problems by just enumerating actor names like Actor001, Actor002. And I will let the garbage collection collect its garbage whenever it desires.