Hello azmom. Welcome to the board. I will try to state it as simple as I can. The underlying cause has to be faced. Generally the triggers that cause the thoughts have to be found. Then control over triggers and their responses has to be modified.
However there is normally nothing simple about stopping. Professional help should be obtained as soon as possible. Try to find someone with experiance in this area. They will normally work with the patient and help them to stop the SI while they are looking for the causes. Take care.