Gregory the Great — on 2Thess 3:15 (The Book of Pastoral Rule, Part 3, Chapter 22)

“Whence the same apostle again admonishes his disciples, saying, "If any man obey not our word, note that man by this epistle; and have no company with him, that he may be confounded." And straightway he added, "Yet count him not as an enemy, but reprove him as a brother." As if to say, Break ye outward peace with him, but guard in your heart's core internal peace concerning him; that your discord with him may so smite the mind of the sinner that peace depart not from your hearts even though denied to him.”