1 Corinthians 5:10

However, Paul hastened to clarify that in writing what he had he did not mean a believer should never associate with fornicators outside the church. He did not mean either that they should avoid contact with unbelievers who were sinful in their attitudes and actions toward people and God. Even our holy Lord Jesus Christ ate with publicans and sinners. Such isolationism would require that they stop living in the real world and exist in a Christian ghetto insulated from all contact with unbelievers. This approach to life is both unrealistic and unfaithful to God who has called us to be salt and light in the world (Matthew 5:13-16; Matthew 28:19-20). Many Christians today struggle with an unbiblical view of separation that tends more toward isolationism than sanctification.

Some interpreters view this discipline as excluding the offender from the community of believers gathered for worship: excommunication. [Note: E.g., Fee, The First . . ., p. 226.] Others view it as social ostracism.

"The Apostle is not thinking of Holy Communion, in which case the mede ["not even"] would be quite out of place: he is thinking of social meals; ’Do not invite him to your house or accept his invitations.’" [Note: Robertson and Plummer, p. 107.]

In 2 Thessalonians 3:14 Paul used the same phrase (Gr. sunanamignusthai, lit. mix up together), translated "to associate with" (1 Corinthians 5:9), with regard to busybodies in the church. There not associating was to be the last resort of faithful believers in their social dealings with their disobedient brethren (cf. 1 Thessalonians 4:11-12; 1 Thessalonians 5:14). They were not to treat them as enemies, however, but as brothers. Probably Paul had the same type of disciplinary behavior in view here. I tend to think it means excommunication and social ostracism in view of the next verse.