Jude is attacking false prophets, and thus men and demons are interwoven in the context. He indicts these false prophets for three sins:
1. Lust: They defile the flesh, allowing a feeling to take them over the edge into sin.
2. Rebellion: They flout authority in general, but primarily that of Christ. It is hidden in the Greek, but the word authority is really "lordship." It normally refers to Christ and His lordship over us.
3. Disrespect or disregard of spirit beings.
This third sin is interesting because he is saying that it is not that these false prophets will not talk about Satan, but their speech is gratuitous, despising, or denigrating of angelic powers. Their preaching suggests that these demons are not something Christians need to be concerned about. They side-step the issue.
Why would they do that? Because a false spirit is leading them, so they downgrade the existence and powers of demons through their preaching. This is clearly seen in Protestant Christianity, especially the mainline denominations that have gone to the point that they almost universally agree that Satan the Devil and his demons do not really exist. It shows how successful the demons have been in their deceptions.
On the other hand, there are evangelical or Pentecostal groups who talk about demons and Satan in a flippant, dismissive way: "Oh, we're going to put down the Devil tonight!" They say such things in their tent shows as part of their evangelistic campaigns. But what they are doing? They are putting Satan into a position where they seem to have power over him. They are so deceived.
The truth in regard to Satan is somewhere in between. The true church of God will have that truth, and they will understand that, yes, Satan is, he is powerful, but because of God, they do have power over him in that they can reject him and his deceptions. We are not puppets on a string, and he cannot influence us unless we give him the opportunity. If we are spiritually aware and can see him at work, we do not have to submit to him.
Jude is giving us signs to look for in the preaching of false ministers. They will denigrate Satan and his demons, there will be indications of lusts, and they will flout the authority of Jesus Christ.