The Nri were peaceful to the extent that they could, but when forced to the utmost, did engage in warfare as in the end of the 18th century. Meanwhile, their traditions—including the sieges that were instigated when someone broke the rules—were designed such that there would be no bloodshed and no deaths.
A more detailed (compared to the Wiki) overview of the Nri—the authority of which stemmed from the bestowing of titles in the settlement of Nri—is provided by Ebighgbo:
The Nri state system (as analyzed by Onwuejeogwu, 1975, and 1981) and the ritual hegemony were not based on the use of military force, for Nri never had a military or police organization. The Nri system of control was based on the propagation of ritual ideology that Eze Nri had mystical powers and that Nri men and Nri towns were sacred. To achieve this control, Nri titled men spread out all over a large proportion of Igboland, and lived in these settlements, they became the chiefs and priests. ...
Yam, palm produce, cocoyam and vegetables were ritualized in the Ifejioku cult.
Leadership was ritualized in the various title systems epitomized in the o~o title, which was controlled and directed by the king of Nri. called Eze Nri. Similarly, the crude agricultural tools such as the hoe, cutlasses, digging sticks, clubs and the single and double headed iron spears, and swords of various types were all ritualized. It was considered an abomination to spill human blood in violence on the surface of the earth. Thus earth was ritualized as a supernatural force called ana and ajana.
—Ebighgbo, 'What Technology Produced, the Igbo-Ukwu Bronzes'
Ebighgbo also describes how these 'titled' men carried otnnvi or ngqucigiiiga, the spear of peace, which acted as their staff of office and represented that they'd been given office in Nri. This symbol (along with others) allowed the bearers of these symbols to move from one settlement to another within the cultural sphere of Igboland. In these journeys, these titled men distributed knowledge, officiated over administrative affairs, and carried out other duties. Oddly enough—for a pacifistic society—one of these duties is noted as "the making of peace". My interpretation of this within the rest of the text is that this refers to peace between other settlements in Igboland, and not peace between Nri and someone else.
Masquerades (or plays) are an important tradition in the area, as well as a potential source for (ritualized) violence. However, it looks as if these engaged the settlement in activity, and while bloodshed is avoided damage to property does not violate the rules:
The mask makes a circuit of the village, accompanied by one or two gong players and a few personal attendants in ordinary dress. It moves with arms held out from the body, and walks with a powerful and heavy tread, threatening onlookers with the knife that it carries, and searching the area with deliberate movements of head and eyes for possible enemies. At times the group of runners who announce the approach of the mask come near the figure and call encouragement in a shout that becomes an invocation, to which the mask responds with a burst of activity, shaking its body furiously, stamping, and rushing at spectators until it is restrained by its attendants. Onward movement through the village is an essential part of the mime, and in a successful performance the spectators rush to follow the mask and so heighten the atmosphere of alarm and excitement. ...
Among senior masquerades the ijele is the only one which forms a public spectacle and which is not associated with violence; all the others in this category rely on feats of strength or of magical skill, and they are regarded as the most fearsome of all masquerades. Their appearances disrupt normal communal life by creating a state of siege and causing much damage to property, and since these excesses are not tolerated under modern conditions the masquerades are rapidly falling into disuse.
—Boston, 'Some Northern Ibo Masquerades'
This article focussed on a slightly larger cultural area as I understand it, but I'm still reluctant to take the violence mentioned above at face value. Beyond the "damage to property", another type of violence mentioned is whipping but with the disclaimer that 'comparatively little real beating takes place'.
The ritual sieges came into play when someone actually shed blood (i.e., desecrated ana). Due to this aversion to bloodshed, wrestling was a common form of settling disputes. However, when a wrestling match—or other accident—caused bloodshed or death, the lineage, settlement, or grouping of settlements which carried it out would find themselves 'besieged'. The siege meant that no trade would go to the people who broke ana's trust, and this siege could be lifted by a representative of Nri conducting rituals of purification.
However, not all the trying in the world prevented all bloodshed—and all war. The Nri's idealistic stances did not prevent them from defending themselves in the utmost circumstances (though one might wonder how successful they were at it).
At the end of the eighteenth century, Igbo - also these which are not belonging to the Nri society - tried in vain to mark themselves inviolable for slavery. At the same time, the Nri were so cornered that they had to resort to the utmost means: they waged a war against the Aro, called Anakom (defense of the land).
—Willendorf, 'Fighting for freedom: From Nigeria to Germany'
The entirety of the 18th century seems to have been a very troubling time for the Nri, with their Eze Nri enslaved, land suffering droughts, social violence, and calamities. Willendorf refers to these, but I couldn't find more detail due to the Books' Preview.
Other Potential Sources
Lastly, a fictionalized account of life in the Nri state is described in 'Nri Warriors of Peace' by Chikodi Añunobi.