Whose mouths must be stopped (οὓς δεὶ ἐπιστομίζειν)
Lit. whom it is necessary to silence. Ἑπιστομίζειν, N.T.o. olxx. Originally, to put something into the mouth, as a bit into a horse's mouth. Ἑπιστόμιον is the stop of a water-pipe or of a hydraulic organ. Comp. φιμοῦν 1Ti 5:18.
Who subvert (οἵτινες ἀνατρέπουσιν)
The double relative is explanatory of must; in as much as they, etc. For subvert rend. overthrow. See on 2Ti 2:18.