The Lord is long-suffering, and those who speak in His name should be also. There do come times, however, when further witness becomes useless or even harmful, or when continued interaction merely invites contamination with ungodliness. In such cases we must simply leave such people alone, following them with prayer and trusting God alone to deal with them.
Such was the ten-tribe nation of Israel, led by the tribe of Ephraim, just before God sent them into Assyrian captivity. God, through the prophet Hosea, told Judah henceforth to let them alone--they were hopelessly given over to pagan evolutionist idolatry. The words "joined to" in today’s verse mean, literally, "under the spell of."
The Lord Jesus used similarly harsh language in reference to the hypocritical Pharisees of His own day: "Let them alone: they be blind leaders of the blind. And if the blind lead the blind, both shall fall into the ditch" (Matthew 15:14).
There are other similar warnings. Of those who come, "having a form of godliness, but denying the power thereof," Paul says, "from such turn away" (2 Timothy 3:5). He has also warned us to "shun profane and vain babblings |that is, the empty philosophizing of those who reject God|: for they will increase unto more ungodliness" (2 Timothy 2:16). "And have no fellowship with the unfruitful works of darkness, but rather reprove them" (Ephesians 5:11).
Most, if not all, such warnings seem in context to apply especially to people who once knew and understood the truth, perhaps even professing to accept it for a time, and then knowingly rejected it. When such men oppose our testimony, God says to let them alone; He can deal with them better than we. HMM