Jamieson, Fausset and Brown on Galatians 5:12

12. they ... which trouble you--Translate, as the Greek is different from Ga 5:10, "they who are unsettling you."

were even cut off--even as they desire your foreskin to be cut off and cast away by circumcision, so would that they were even cut off from your communion, being worthless as a castaway foreskin (Ga 1:7, 8; compare Php 3:2). The fathers, Jerome, Ambrose, Augustine, and Chrysostom, explain it, "Would that they would even cut themselves off," that is, cut off not merely the foreskin, but the whole member: if circumcision be not enough for them, then let them have excision also; an outburst hardly suitable to the gravity of an apostle. But Ga 5:9, 10 plainly point to excommunication as the judgment threatened against the troublers: and danger of the bad "leaven" spreading, as the reason for it.