A.T. Robertson Commentary Mark 1:34

"And he healed many that were sick with divers diseases, and cast out many demons; and he suffered not the demons to speak, because they knew him." — Mark 1:34 (ASV)

Devils (δαιμονια). Demons it should be translated always.

Suffered not (ουκ ηφιεν). Would not allow, imperfect tense of continued refusal. The reason given is "because they knew him" (οτ ηιδεισαν αυτον). Whether "to be Christ" (Χριστον εινα) is genuine or not, that is the meaning and is a direct reference to 1:24 when in the synagogue the demon recognized and addressed Jesus as the Holy One of God. Testimony from such a source was not calculated to help the cause of Christ with the people. He had told the other demon to be silent. See on Mt 8:29 for discussion of the word demon.