Paul gives his command by the authority of the Lord Jesus Christ (1 Thessalonians 4:2).
Church members should avoid Christians who are lazy (see 2 Thessalonians 3:7, 11–13; 1 Thessalonians 5:14). Staying away from undisciplined people would strongly impact them (Matthew 18:17; Romans 16:17; 1 Corinthians 5:9–13). This would promote shame and repentance (2 Thessalonians 3:14), but it would stop short of removal from the church (3:15).