Even if everybody agreed that it was immoral; and wrong and shouldn't be allowed, it's unlikely it could be stopped.
International organisations with sufficient power can influence national governments sometimes with economic sanctions.
Consider how long the South African Apartheid system persisted. It might possibly have collapsed faster if there were not governments willing to support it.
Consider how ineffective world opinion has been over Russia's invasion of parts of Ukraine. EU economic sanctions have been enough to see the country fall into economic recession, but not enough to change the dictator's mind or reduce public support.
Consider how 1930's Germany treated minorities. It was condemned but considered to be an internal problem and nobody else's business. War broke out only due to traditional mutual defence treaties. There was great doubt later that the Nuremburg trials could be legal.
The powerful USA strong-arms weaker countries into promising that they will never take action against it in the International Criminal Court even if, for example, their forces abuct, torture and murder the small county's citizens.
The UK government intends to withdraw from the European Convention on Human Rights which the UK drafted and foisted onto other European countries. If you are unlucky enough to be sold into slavery, or your belongings are seized, or the Sun taps your phone and bugs your home, and the UK government decides not to help you, for example because you are Welsh or a Methodist or because Rupert Murdoch chooses the Prime Minister, there will be no-one else you can appeal to.