Is there any disease that prevents a person from being angry? Is there any disease that prevents a person from being angry? As a reference, the resection of the amygdala reduces feelings of fear.

web links  


ABC news on damage to the amygdala reducing fear
Anecdotal report on inability to feel anger
Lack of expression of anger
Short answer
Brain damage is typically associated with increased expression of anger and aggressive behavior. I wasn't able to find any paper reporting lack of anger expression after brain damage. Great question!  

Background
Assuming you are after symptoms associated with brain damage, I have put my efforts into writing up an answer on brain trauma and anger. To strat with your example of the amygdala - indeed, removal of the amygdala (amygdalectomy) has been used to treat extreme feelings fear. Damage to the amygdala reults in deficits in facial emotion recognition. Specifically they have impaired recognition of expressions of fear and guilt and trustworthiness. These findings suggest that the amygdala has a critical role in face emotion processing (Elliott et al., 2011).

Anger is thought to be mediated by the medial amygdaloidal areas that connect to the medial hypothalamus via the stria terminalis, and from there to the dorsal half of the periaqueductal gray. Aggression (the expression of anger) evoked by stimulation of the amygdala is dependent on the functional integrity of the medial hypothalamus and the periaqueductal gray. However, aggression evoked by stimulation of the periaqueductal gray is not dependent on the functional integrity of the amygdala. The amygdala-hypothalamus-periaqueductal gray is thought to mediate reactive aggression (Blair, 2013).

Hence, it seems that a likely candidate for the obliteration of feelings of anger is the periaqueductal gray. Does damage or removal of this structure reduces feelings of anger? Lesions in this area of the brain have been associated with loss of speech in humans (Esposito et al, 1999), reduced fear in rats (Lonstein &amp; Stern, 1997), and upon complete destruction it resulted in a limp inertness in cats (Bailey &amp; Davis, 1943). The Lonstein &amp; Stern (1997) paper reports that 


  [Periaqueductal gray] lesioned rats attacked a strange male twice as often as controls did, which is suggestive of reduced fearfulness. These results extend the known roles of the PAG [periaqueductal gray] in [] defensive behaviors. 


Hence, removal of PAG actually increases aggressive behavior (reduces fear), rather than decreasing aggressive behavior! 

In all, I wasn't able to find any study that reports on brain damage that results in a reduction of anger expression. Indeed, brain damage is often related to increased feelings of anger and there the track record is extensive   

References
- Bailey &amp; Davis, *Proc Soc Exp Biol Med (1942); 51: 305-6
- Blair, Wiley Interdiscip Rev Cogn Sci (2012); 3(1): 65–74
- Elliott et al., Neuropsychopharmacology (2011); 36(1): 153–82
- Esposito et all., NeuroReport (1999); 10(4): 681–5
- Lonstein &amp; Stern, J Neurosci (1997); 17(9): 3364-78