Theophylact of Ohrid

Moreover, He rebuked the spirit upon seeing the crowd gathering toward Him — again because He did not wish to heal before a multitude of people, for the display of His power and for His own glory. And by the rebuke and the words, "Come out of him and enter him no more," He makes known that because of the man's unbelief the demon could have entered him again, had it not been forbidden by His command. …

Commentary on Mark ↗

Bede

And when Jesus saw the crowd running together, he rebuked the unclean spirit, saying to it: You deaf and mute spirit, I command you, come out of him, and never enter him again. The rebuke of the Lord is the power of the divine command. But he rebuked not the boy who was suffering harm, but the demon who was inflicting it. For whoever desires to correct a sinner must indeed exterminate the vice by rebuking and cursing it, but nourish the man by loving him. …

On the Gospel of Mark ↗