Those thrown into the lake of fire will be turned into ashes (Ezekiel 28:18, Malachi 4:3). In the HBFV translation of Matthew 5 it states Jesus used the constant flames in the valley of Hinnom (Gehenna) to symbolize the destructive nature of God's judgment upon all those who indulge in wickedness.
28. Do not be afraid of those (other humans) who kill the body, but do not have power to destroy the life; rather, fear Him (God) Who has the power to destroy both life and body (their entire existence) in Gehenna (lake of fire).
Can spirit beings really be destroyed?
Demonic spirits are fully aware of their fate, brought by fire, which awaits them. In what would become Christ's first recorded casting out of a demon, an evil spirit recognizes him during a synagogue service in Capernaum. The spirit, no doubt surprised that God's Son was preaching to the congregation, cries out the following before being commanded to leave.
"Ah! What have we (the demons) to do with You, Jesus, the Nazarene? HAVE YOU COME TO DESTROY US? I know Who You are - the Holy One of God!" (Mark 1:24, HBFB throughout)
18. By the multitude of your iniquities, by the unrighteousness of your trade, you have profaned your sanctuaries; therefore I brought forth a fire from your midst; IT SHALL DEVOUR YOU, and I will bring you to ASHES upon the earth, before the eyes of all who see you. 19. All who know you among the people shall be astonished at you; you became a terror, and YOU WILL NOT BE ANY MORE (Ezekiel 28:18 - 19).
Whatever God creates he can destroy.
Timeline of destruction
According to Scripture, the lake of fire is first used at the time of Christ's Second Coming. The book of Revelation states it will be used to destroy the end time Beast and False Prophet who deceived the whole world.
20. And the beast was taken, and with him the false prophet who worked miracles in his presence, by which he had deceived those who received the mark of the beast and those who worshiped his image. Those two were cast alive into the lake of fire, which burns with brimstone (Revelation 19:20).
Its next use occurs shortly after the devil, at the end of Jesus' Millennial (1,000 year) reign on earth, will be set loose from prison and allowed to deceive the world one last time (Revelation 20:7 - 9). For at least the third time he will muster an army in order to wrest control of the universe away from the Eternal. His attempt, however, will utterly fail. He will be caught and tossed into the fiery lake where his life will end (Revelation 20:10).
Although the Bible is unclear regarding when demons will be judged, then receive their penalty, it will probably occur at the same time or shortly after the devil is put out of existence. Although the majority of them will likely be thrown into the lake of fire (see Luke 4:34), Scripture seems to indicate some will be restrained for all eternity (see our article on Satan's fate for more info). The book of Jude alludes to these two kinds of penalties (Jude 6, 7, 13).
God, before He creates a new heaven and earth where only righteousness dwells (Revelation 21:27), will also judge and execute all those humans who willfully reject his generous offer of salvation. This ultimate punishment in the Bible is called the second death, which will occur in the lake.
11. Then I saw a great white throne and the one Who was sitting on it, from Whose face the earth and the heaven fled away; and no place was found for them. 12. And I saw the dead, small and great, standing before God . . . 15. And if anyone was not found written in the book of life, he was cast into the lake of fire (Revelation 20:11 - 12, 15).
The last thing thrown into the lake will be death itself.
26. The last enemy to be destroyed is death (1Corinthians 15:26)
14. And death and the grave were cast into the lake of fire . . . (Revelation 20:14).
God, after death itself is taken out of existence, will use fire to cleanse the entire universe before he creates a new heaven, earth, and even a New Jerusalem.
11. Since all these things are going to be destroyed, what kind of persons ought you to be in holy conduct and godliness, 12. Looking forward to and striving for the coming of the day of God, during which the heavens, being on fire, shall be destroyed, and the elements, burning with intense heat, shall melt? 13. But according to His promise, we look forward to a new heaven and a new earth, in which righteousness dwells (2Peter 3:11 - 13, see also Revelation 21).