The nature of reality is such that there is moral nihilism i.e. there is no objective right or wrong. This gives rise to the principle of "might makes right" where power dictates everything rather than morality. A billionaire who can lobby politicians can easily get away with child rape. Even if someone believes that child rape is wrong or even if someone has empathy for the suffering of the child, the reality is that a billionaire who rapes a child and lobbies politicians can get away with it.
As a result of moral nihilism and "might makes right" this means that morality competes based on power. Negative utilitarians believe that suffering is wrong, that we must minimise suffering as a matter of priority. However, this doesn't make suffering objectively wrong if we assume moral nihilism. It means that negative utilitarians have a subjective preference against suffering. When a negative utilitarian witnesses a child being raped or a person eating meat, the negative utilitarian has a subjective preference against this. Because of "might makes right" then the only way that the negative utilitarian can express his views is through force.
There are two ways the negative utilitarian can force other sentient living organism to not harm others and impose suffering on others: (1) antinatalism to force future generations to not cause suffering and (2) force existing living organism to reduce harm.
Regarding (1) many people think that we should give sentient living organisms the freedom to choose whether they procreate or not because forcing antinatalism causes suffering. However, due to the natural biological instinct to procreate, giving total freedom to a living organism only results in heavy procreation, which results in harm and suffering. Hence there needs to be a degree of force involved. I emphasise degree of force because too much force leads to suffering, which is what we want to reduce in the first place. Hence we should not forcably surgically sterilise others. There are ways we can impose contraception on others e.g. for animals we can use chemical contraceptives, which we put in the food of animals. Animals eating this food suddenly do not feel like procreating. For humans, who can reason, intellectual arguments can be given to convince them to not procreate. Spreading antinatalism can be enough to convince someone to not procreate. Other ideologies can be spread that has the same outcome e.g. spreading homosexuality can reduce fertility rate. Furthermore, certain government policies or economic systems may reduce fertility rate e.g. it is widely considered that female education reduces fertility rate. Econonomic development and urbanisation are widely seen as an effective contraceptives, leading to many countries transitioning from agrarian societies with fertility rates of 5 or 6 to below replacement rate (2.1) within one generation thereby leading to population reduction.
For all sentient living beings, "soft contraception" should be imposed, and everyone should contribute to the imposition of soft contraception on all living beings.
Of course, just because you impose contraception on others and implement antinatalism and therefore prevent future beings from coming into existence and causing or experiencing suffering, it doesn't mean that those who currently live cannot experience or impose suffering on others. Therefore, the solution is the reduction of harm for those who are currently alive, and this should be through the legal system. In fact, the legal system and government is arguable set up for this purpose: to force those who are currently alive to not harm others and therefore to reduce suffering. The existing legal system should be refined so as to reduce suffering e.g. murder should be banned, rape should be banned, and ideally eating meat should be banned.