Paul did not need, nor would he accept, the testimony of the girl towards the crowd on his behalf. And when she kept at this, accompanying Paul in such outspoken and irregular manner, he then acts. He spoke to the demon spirit within her and ordered that spirit out. He commanded the demon spirit to leave the girl in the Name of Jesus Christ.
By using the Name of Jesus Christ the power of darkness and demons flee. You may use His Name like this and the spirit will come out of the other person. And once the demon spirit is out then fill that empty space within her/him with Christ’s love, joy, hope, and peace. Speak Christ’s salvation way to them.
Salvation can now enter into her heart and is not blocked. Jesus Christ is our freeing Agent to use in this way. As Paul has learned this from the Apostles first-hand then he is able to put this into practice. This is a method of Christianity that has to be taught first-hand, person to person, and when you have seen it you are able to use it too.