The stars are not wanted now: put out every one, Pack up the moon and dismantle the sun,
After that devastating line 12, the speaker grows even more mopey in these lines. Reentering imperative land, he demands that someone, whomever he's talking to, put out the stars, pack up the moon, and take apart the sun. Now his grief is so extreme, it's affecting the way he sees the cosmos. This is some serious business.
Does the speaker expect us to really do this? Of course not. But his extreme, hyperbolic commands are his expressions of his extreme grief.
Even though no one could ever "dismantle the sun," the speaker's grief is so intense that he wishes that we could. All of these romantic and natural images—the stars, the moon, the sun—are too painful for him. It's almost as if he wants to blot out everything in the world except his own mourning.
Pour away the ocean and sweep up the wood; For nothing now can ever come to any good.
In these final lines, the speaker continues his hyperbolic thinking and asks us to get rid of the ocean and the wood (by "wood," he probably means the forests). He doesn't want to see any sign of the wonders of nature because he's so down in the dumps.
The last line of the poem is another whammy. Totally hopeless, the speaker mopes that nothing will ever be good again. Not since this guy's death.
In a lot of elegies (poems like this one that commemorate a person's death), the speaker will offer some hope for the future, or will talk about how the dead person will live on in memories and poetry. There's usually a small moment of optimism buried somewhere in them. But not in Auden's "Funeral Blues." This is just a really sad poem about death. There is no light at the end of the tunnel for anyone in "Funeral Blues."