During Lutheran Confession, can the confessor refuse to absolve the sins of the penitent, if he or she thinks that the penitent is not being sincere? What if the penitent is sincere but cannot resist laughing at the many things that he or she has to say and is ashamed of?
First and foremost, nobody - neither Catholic nor Protestant - believes that a priest "forgives" sins. At best, they act as an intermediary, supplicating to Jesus, who alone forgives sin.
As the Cathecism of the Catholic church states:
Whether or not a priest chooses to exercise this role, in any form of Christianity, is up to the priest.
As an intermediary between man and God, Lutherans do not believe that a priest has an absolute lock on forgiveness. Indeed, as with most Protestants, Lutherans do not believe that the priest is doing anything except for incarnating the action that Jesus himself performs in forgiveness.
Put another way, Luterhans would not see the priest as having any particular monopoly on the granting of forgiveness. As a practical matter as well, the priesthood of all believers means that in reality, any believer any seek forgiveness from any other. (Note: While this doctrine is usually associated with Baptists, Luther himself subscribed to the notion, if not the term.)
As such, if a priest didn't pronounce absolution, it would be a trivial matter for the penitent to secure another "priest" who would, or even find his forgiveness directly through prayer with the Lord.
I'm not a theological expert, however, the Lutheranism that I know identifies the confession with the affirmation of a person's baptismal covenant. Confession witnesses to our faith in God to forgive and make us new in the new covenant with Jesus. It would be very improper for a Lutheran to go to communion without identifying themselves (i.e. confession) as a sinner that sincerely desires to change. My understanding is that those that are not sincere (and only God know's our hearts) reject the covenant and risk having their heart progressively blinded to God's truth. The people that are sincere, should find that the gift of communion causes real action toward being a new person.