And John answered him, saying, Master, we saw one casting out devils in thy name, and he followeth not us: and we forbad him, because he followeth not us.
Note 10 at Mr 9:38: Mark recorded this instance of one casting out devils between his illustration of the little child and his statement about offending one of these little ones, while Matthew omitted it. Mark's inclusion of this instance in the context gives a broader application to Jesus' statement about offending one of these "little ones" (see note 2 at Mt 18:6). It no doubt included the little child He had used when making His point, but it also was a warning against coming out against any brother in the Lord because he isn't doing things our way (such as this man who was casting out devils in Jesus' name but was not following the disciples).
There is a place for discipline in the body of Christ (see note 5 at Mt 18:17 and note 6 at Mt 18:18), but we must be sure that we are enforcing the Word of God and not our own convictions. The Scripture didn't say this man wasn't following Jesus. The disciples said he wasn't following them (this verse), or following with them (Lu 9:49). This man wasn't following Jesus as His twelve disciples were. He wasn't physically traveling with them, but he was following Jesus' teachings and examples. He was doing the works of Jesus (Joh 14:12).
We must be careful never to haggle over methods. There are different ways of administrating and operating in the same gift (1Co 12:5-6). Any judging or disciplining must be administered by the direction of the Holy Spirit and not by our own imperfect standards (see note 46 at Mt 7:1).