While it is true that hallucinations often come and go at their own pace, clinical practice indicates that this is not a hard and fast rule. On the basis of that empirical finding, a range of practical courses have been developed that teach individuals how to gain a certain degree of control over their hallucinations, and how to 'zap' those with a negative content. The effectiveness of these methods has been documented extensively. Although the results are not univocal, the studies do indicate that the frequency, duration, and content of hallucinations are more amenable to conscious control than has traditionally been assumed. See also the entry Cognitive therapy and hallucinations.
References
Coleman, R., Smith, M. (1997). Working with voices. Victim to victor series no. 1. Gloucester, MA: Handsell Publishing.
Jenner, J.A., van de Willige, G. (2001). HIT, hallucination focused integrative treatment as early intervention in psychotic adolescents with auditory hallucinations: A pilot study. Acta Psychiatrica Scandinavica, 103, 148-152.
Dictionary of Hallucinations. J.D. Blom. 2010.