We see Jesus authorizing St. Peter in Mtt 18:18:
Truly I tell you, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.
But, we also see Jesus telling in Mtt 12: 31-32:
And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
Catholics look at the confessional as a place where they can seek and get forgiveness for their sins. But there are situations in which the priest at the confessional can deny to stand as a mediator for absolution to the person confessing before him. My question therefore is: According to Catholic Church, which are the situations in which the priest at the confessional can refuse to stand as a mediator of absolution?