Ridding oneself of the effects of occult activity can be very complex depending on how deeply a person became involved in the dark arts. The case described by DB involves using a pendulum - which is a device used for divination. When we use this device, we are calling upon demonic powers to "divine" whatever it is we're looking for, and these powers don't just leave when we decide we've had enough. They have a mind of their own and will continue to hang around for as long as they are not forced to leave.
How do we force them to leave?
First, as you indicate, you must personally renounce the activity by saying something like, "Jesus, you are my only Lord and Savior. I renounce the use of the pendulum and ask your forgiveness for this sin."
Second, this must be confessed in the Sacrament of Reconciliation. As the famous exorcist, Father Gabriele Amorth writes, confession is the best exorcism and is the most direct means to fight Satan. " . . .(I)t is the sacrament that tears souls from the demon's grasp . . . I advise frequent confession, possibly weekly, to all victims of evil activities." (An Exorcist: More Stories, pg. 195)
If we've done this and are still experiencing oppression or any other physical or spiritual effects such as (aversion to sacred objects/prayer/sacraments, obsessive thoughts, groundless fears and anxieties, etc.) this could mean that bonds were formed between you and the demonic during this activity that now need to be broken.
If this is the case, my first recommendation is to see a priest who is well-versed in the occult. These priests are often affiliated with the charismatic renewal and can be located by going to a local Catholic charismatic prayer meeting, explaining your situation, then asking for a priest to contact. If there is one in your area, they will know how to reach him.
However, if you are unable to find a priest, Father Amorth, lists the recitation of our baptismal promises on a daily basis as a way to break these bonds. (An Exorcist: More Stories, pg. 194)
You can also find several powerful prayers for deliverance in Father Amorth's first book, An Exorcist Tells His Story, beginning on page 199.
The Catholic Warrior also has prayers available on-line and in booklet form that can help you to break these bonds.
If you have any questions at all about the dark arts and how to protect yourself from their influence, read this excellent pastoral letter by Bishop Donald W. Montrose, entitled Spiritual Warfare: The Occult Has Demonic Influence .
This letter is easy to read and offers plenty of advice about what to do if you've been involved in the occult, and how to protect yourself and your family from the influence of Satan.