Dear Eric:

Sacramentals are not magic, they are not talismans. Such things are the works of the devil. Sacramentals cannot "prevent" anything or cause anything in-and-of-themselves.

Sacramentals are gifts from the Church as ...

Objects or actions that the Church uses after the manner of sacraments, in order to achieve through the merits of the faithful certain effects, mainly of a spiritual nature. They differ from sacraments in not having been instituted by Christ to produce their effect in virtue of the ritual performed. Their efficacy depends not on the rite itself, as in the sacraments, but on the influence of prayerful petition; that of the person who uses them and of the Church in approving their practice. The variety of sacramentals spans the whole range of times and places, words and actions, objects and gestures that, on the Church's authority, draw not only on the personal dispositions of the individuals but on the merits and prayers of the whole Mystical Body of Christ. (Source: Fr. John Hardon's Modern Catholic Dictionary)

To answer your questions directly:

1)  Does wearing a scapular prevent one from becoming possessed?

No. Living the Christ-Life in a state of grace is what can prevent possession. For all practical purposes, live the Godly life, in the Sacraments of the Church, and you need not worry about such things.

2) Does wearing a scapular guarantee that a person will not go to hell?

No. Living the Christ-Life in a state of grace, dying in a State of Grace, guarantees a person will go to heaven. No Sacramental can guarantee anything. It is our Faith that give us the hope of eternal life not some object.

3) What protection from the devil does the scapular provide?

What the definition above is saying is that the power of the sacramental is not in the object, but in the prayers and devoted life of the person who uses the sacramental. Using the sacramental helps the person to live the Christ-Life. It is in living the Christ-Life in a State-of-Grace that evil is warded off and that we may die in that State-of-Grace and thereby go to heaven.

As blessed objects sacramentals may ward off some demons as demons generally do not like to be around blessed objects, but there is no guarantee of this as some demons are stronger than others. Sacramentals are like No Trespassing signs — some demons will be scared off by the sign, others will ignore the sign and trespass anyway. Sacramentals are not magic bullets.

Sacramentals have no power of their own, but are signs of our faith. Living out our Faith in love of neighbor, in our devotion, worship, and love of God, participating in the Sacraments of the Church, gives us the best protection against evil and gives us the assurance of the hope of eternal salvation.

But, there is never a guarantee that evil will not harass us or come into our lives even if we live a holy life. God, in his wisdom, may allow evil to visit us to test us, or strengthen us, or to mold us into holy saints. The Saints were all attacked by the devil.

The key to remember is to trust God and that he will alway make sweet lemonade from any lemons in our lives if we let Him (Romans 8:28). Also, God will never allow any temptation to come into our lives that we cannot resist. He will always give us a way to escape the temptation, if we will only take that escape route (1 Corinthians 10:13).

Trust in our Lord, keep you eyes on the Jesus and not on the storms in your life, and be not afraid. Use Sacramentals as powerful signs of our faith in the One who is All-Powerful, our Lord Jesus Christ.

God Bless,
Bro. Ignatius Mary