Quotes suggesting the "unpardonable sin" is simply refusing to repent (even in the next life):

From Joseph Smith:

All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him. After a man has sinned against the Holy Ghost, there is no repentance for him. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it ...

From Joseph F. Smith:

No man can possibly commit the unpardonable sin in ignorance. A man must be brought to a knowledge of Christ; he must receive a testimony of Christ in his heart, and possess light and power, knowledge and understanding, before he is capable of committing that sin. But when a man turns away from the truth, violates the knowledge that he has received, tramples it under his feet, puts Christ again to open shame, denies His atonement, denies the power of the resurrection, denies the miracles that He has wrought for the salvation of the human family, and says in his heart, “It is not true”, and abides in that denial of the truth, after having received the testimony of the Spirit, he commits the unpardonable sin.

From Spencer W. Kimball:

In the realms of perdition or the kingdom of darkness, where there is no light, Satan and the unembodied spirits of the pre-existence shall dwell together with those of mortality who retrogress to the level of perdition. These have lost the power of regeneration. They have sunk so low as to have lost the inclinations and ability to repent, consequently the gospel plan is useless to them as an agent of growth and development.