They will suffer the punishment of eternal destruction, separated from the presence of the Lord and from his glorious might. 2 Thess. 1:9, T.E.V.
If you have a table or desk lamp near you as you read this, let your eyes glance from the bulb down to the power cord at the base of the lamp. Then trace the cord across the floor to the wall plug. From there you will have to imagine electrical wires running through your house, across town, over the countryside, and finally ending at some huge generating plant. The little bulb in your lamp gives off light because it has an unbroken connection with the power plant. If the wires are broken at any point--by a tree falling across the power lines, or by you pulling the plug--the light will go out. The bulb cannot give light by itself.
In the same way, we live because we are in union with the Life-giver, our heavenly Father. Though we have broken connection with Him, God has been giving us the daily grace-lift of life that we might choose to renew our loyalties with Him. But He will not forever keep the whole planet on an emergency life-support system. God has great respect for our freedom; the day will come when those who refuse His gracious invitation for friendship will be given what they have chosen: separation from Him.
When you unplug your lamp, it doesn't explode. The light just goes out. Nor do you need to beat on the bulb in anger for its ceasing to give light. That's simply what happens when it is disconnected. By the same token, when one breaks union with God life ceases. God does not, in anger, need to crush it out. Nor does He need to work a miracle to keep a person alive so that He might at the same time inflict unending punishment on him. To be separated from the Life-giver is to be dead--eternally.
The people God was addressing in Biblical times did not always understand this cause-effect principle. It was difficult for them to appreciate the destructiveness of being out of harmony with God. And so the Bible writers employed the imagery of consuming flames to describe the sureness and completeness of the destruction of life apart from God. But being apart from God is in itself the worst thing that could ever happen to a person. God doesn't need to torch hellish fires to enhance what is already so terrible.