The entire poem is about a prayer that a person offers up to God, asking him to give them everything-all of life's experiences, and to spare them nothing so that they can experience it all. In the sixth stanza, the Lord is answering that prayer. Before this, he has stated that he will do just that, that their soul will "know all passionate rapture and despair...drink deep of joy and fame...love shall burn thee like a fire, and pain shall cleanse thee like a flame." So the Lord will answer her request, and let her experience it all-good and bad.
Then, he states, "So shall thy chastened spirit yearn/To seek from its blind prayer release,/And spent and pardoned, /sue to learn/The simple secret of My peace." In this stanza he says that her spirit, through these experiences will be chastened or humbled, and she will wish a release from the prayer she offered-a release from all of these experiences. Her soul, "spent and pardoned", or tired and exhausted, will want to know how to be peaceful. It will want to know how to leave the fire and flame behind, the burning and cleansing, and simply experience quite, underrated peace.
All in all the poet will learn that all of the drama and excitement in life is not necessarily what makes it good; sometimes, it is the peaceful, quiet moments, and in these, we are closest to God. I hope that helped!
So shall thy chastened spirit yearn
To seek from its blind prayer release,
& spent and pardoned, sue to learn
The simple secret of My peace
Before this verse the poem has been about how the poet prays to God to feel everything in the whole world, all life's joys and pain at the most intense levels. And God says to the poet, "I will give you what you ask for." And God promises to send the poet many intense emotional experiences both good and bad.
Then God says to the poet, after you have experienced all the love, joys and highs and lows of life, your soul will not be satisfied and it will beg me to undo its silly request (blind prayer). And, tired and forgiven, your soul will beg to learn about peace, instead of intensity.