—2 Thessalonians 3:6 (NASB)
We need to read this carefully and make special note of who exactly Paul is referring to. He's not advocating that we stay away from everyone, but only the "brother who leads an unruly life". Whenever Scripture references a brother it is referring to someone who's supposed to be a fellow believer. Paul is providing not just an alternate definition of a hypocrite, but is advising that to associate with those who are obedient in word only places our own faith at risk.
But we also need to consider that this is necessary not just for our own sake, but for our brother's as well. As long as we allow their behavior to go unaddressed we are lending a kind of tacit approval. To sever fellowship with them entails a discussion that must speak the truth about why we can no longer associate with them. In most cases this will probably lead to their undertaking the necessary change; if they don't respond, we certainly can see how little they think of both their relationship with Christ and with us.
We're not talking about someone who occasionally sins or has a bad moment or two, but is engaged in a lifestyle that is contrary to the biblical definition of what it means to be a Christian. The best thing we can do both for them and ourselves is to enact a kind of "spiritual intervention" which in parting ways categorically states the truth concerning the contradiction of their life. And if you'll take note, this is not a request which is made of us, but a "command". There's no room for tolerance of this kind of lifestyle because tolerance can't restore them to the Lord; only faithfulness can heal unfaithfulness.†††