Greek: τοὺς δὲ ἑτεροκλινεῖς ὑπάρχοντας εἰς κόλασιν καὶ αἰκισμὸν τίθησιν
The version of Lightfoot that I'm using translates this passage: "but appointeth unto punishment and torment them which swerve aside."
δὲ = "but"
τοὺς = "them"
τίθησιν = "appointeth"
εἰς κόλασιν καὶ αἰκισμὸν = "unto punishment and torment"
ἑτεροκλινεῖς ὑπάρχοντας = "which swerve aside"
I would probably not change much about Lightfoot's rendering. Maybe "but set [placed, assigned] to punishment [pruning, mutilation, cutting-off] and harm them which are inclined to other loyalties."
1. κόλασιν can denote "punishment" (BDAG, Robertson's WP). See http://www.biblestudytools.com/commentaries/robertsons-word-pictures/matthew/matthew-25-46.html as an example. While I do not agree with Robertson's overall explanation for Mt 25:46, I believe that the Greek word may denote "punishment" which is not the same as the verb "punishing." See 1 John 4:18. BDAG includes the definition "retribution" as well.
2. BDAG points out that when αἰκισμὸν is used with κόλασιν (or their related lexical forms) that αἰκισμὸν evidently means "punishment." It cites 1 Cl 11:1. So what we appear to have in this passage is a redundancy or pleonasm. The words essentially communicate the same idea.
3. BDAG also cites 1 Cl 11:1 for the entry ἑτεροκλινεῖς. The word may convey notions of "leaning to one side" or "inclined to, having a propensity for" but in 1 Cl 11:1 probably means "having other allegiance" since it's used in the context of rebellious or apostate peoples.
By the way, ὑπάρχοντας is a present active participle ("being") in the accusative form.
Hope this helps,