Note 3 at Acts 16:18: Paul did not cast this demon out immediately. He waited many days. Just as in the case with Elymas, the sorcerer, it could have taken Paul some time to perceive where this girls faith was (see note 6 at Acts 13:9, p. 648). There also is no mention that this girl became a convert after her deliverance, so he may have been hesitant to cast the demon out, knowing that without Christ in her life, the demons would just come back worse (see note 4 at Mt. 12:44, p. 190).

Also, apparently the Lord does not intervene in an individual's life against their will except in judgment (see note 5 at Acts 13:9, p. 648). Once a person comes under the punitive judgment of God, there is no repentance (Rom. 2:4). Therefore, Paul may have delayed casting this demon out, hoping that this girl would respond positively to the gospel and be born again.

When he perceived that she was not going to change but that instead she was turning others away from the gospel, he was forced to intervene. This may be the reason he was "grieved" (see note 4 at this verse).

Note 4 at Acts 16:18: There could be different reasons why Paul was grieved. Paul could have been grieved because of the way this girl's actions and words were turning others away from faith in Christ. It's also possible that he was grieved for the girl's sake.

He had delayed casting this spirit out for many days, not because he didn't have the power to do so, but because under the new covenant, God does not judge His people in a punitive way and apparently only judges unbelievers as a last resort (see note 4 at Acts 12:23, p. 646). Paul knew that by casting this spirit out against this girl's will, there was little or no chance of her ever repenting and coming to the Lord and that grieved him. Yet, it had to be done for the sake of the others who she was seeking to turn away from the faith.

Note 5 at Acts 16:18: Paul spoke to the spirit, not the girl (see note 2 at Mk. 8:33, p. 264).

Note 6 at Acts 16:18: The scriptures say the spirit came out the same hour, not immediately. It may have only been a few moments of that hour but this does establish a precedent that there can be some time in-between when a person commands the devil to come out and when he actually leaves. There is no indication that Paul continued to command the spirit to come out more than once.