They is a general word, “such people” as Paul has just described.
The word for punishment normally means a penalty imposed by a court of law.
The word here rendered suffer is less emphatic than the unrelated words for “suffer” which Paul uses in verses 5 and 6. It means simply “undergo.” Moffatt translates “they will pay the penalty.”
Eternal destruction is a difficult phrase, not only from the theological point of view, which does not concern us directly here, but also for the translator. Destruction suggests that the persecutors will cease to exist, but if this were so, how could their destruction be eternal except in the sense of “final” or “permanent”? This same word for destruction could be translated “disaster” in 1 Thess. 5.3, and New American Bible translates it “ruin” here. Eternal is often used in the New Testament in speaking of the age to come or the last days. “Eternal life” is not simply life which goes on forever, but life of a quality associated with the age to come. Eternal destruction is the opposite of this. In 1 Thess. 4.17 Paul describes the life of the age to come by saying simply “we will always be with the Lord.” Conversely, eternal destruction is to be separated from the presence of the Lord. The persecutors will be punished in such a way that “all that makes life worth living is destroyed” (H. A. A. Kennedy, quoted by Best, p. 262).
Eternal destruction must be rendered in some languages as a verb phrase, for example, “he will destroy them forever.” This expression may be related to what precedes as “they will be punished; God will destroy them forever,” or “God will cause them to suffer; he will punish them by destroying them forever.” Note, however, that this eternal destruction is further defined by the phrase which follows, namely, separated from the presence of the Lord.
Separated from represents a common Greek preposition which has a wide range of meanings. King James Version Moffatt Zürcher Bibel translate it rather vaguely as “from.” Knox thinks that it is God’s presence which causes the destruction and translates “the presence of the Lord, and the majesty of his power, will condemn them to eternal punishment,” but he adds a note giving another interpretation. It is more natural to understand the word “from” as referring (metaphorically) to space, and this is the meaning in Isaiah 2.10, 19, 21, of which this verse is a quotation. Most translations understand in this way: separated (cf. Die Bibel im heutigen Deutsch), Jerusalem Bible “excluded” (cf. Phillips), New English Bible “cut off,” Barclay Translator’s New Testament “banishment” (cf. Bijbel in Gewone Taal), “far from” (cf. Bible en français courant Biblia Dios Habla Hoy Luther 1984 Bible de Jérusalem La Sainte Bible: Nouvelle version Segond révisée Le Nouveau Testament. Version Synodale Zürcher Bibel).
The phrase separated from the presence of the Lord serves to further define the meaning of eternal destruction. One may therefore introduce this expression as “this means they will be separated from the presence of the Lord.” In some languages, however, it is extremely difficult to use a passive expression such as separated without indicating precisely who does the separating and the mode of the separation. One must say in some instances “God will close them off from where the Lord will be.” In many instances, of course, the Lord must be rendered as “our Lord.”
The presence of the Lord is a non-figurative equivalent of the Greek “the face of the Lord,” which is not very natural in current English. The Lord, as usual in Paul’s writings, means Jesus (cf. v. 8), though in the passage in Isaiah which Paul is quoting, the Lord naturally refers to God. His glorious might is literally “the glory of his might,” and could also mean “the glory which comes from” or “is caused by his might.” On “glory,” see 1 Thess. 2.12; cf. 2.6, 20. In some translations his glorious might has been rendered merely as “his wonderful strength.” But this is often not satisfactory. How can one be separated from someone’s strength? It is possible in some instances to reverse the attribution and to speak, not of “glorious might,” but “great wonderfulness”; that is to say “God will shut them away from the great wonderfulness of our Lord.” In some cases this may be best indicated by “and they will never see how very wonderful he is,” or “… never experience his great wonderfulness.”
Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on Paul’s Second Letter to the Thessalonians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .