Adam Clarke on Galatians 1:9

Let him be accursed - Perhaps this is not designed as an imprecation, but a simple direction; for the word here may be understood as implying that such a person should, have no countenance in his bad work, but let him, as Theodoret expresses it, Αλλοτριος εστω του κοινου σωματος της εκκλησιας, be separated from the communion of the Church. This, however, would also imply that unless the person repented, the Divine judgments would soon follow.