It’s called patam nikkujjana kamma, literally “overturning the bowl”. It signifies that the monks will not accept alms – in this case, from the officials of the Ministry of Justice, which had imprisoned activists and monks. It’s the Buddhist version of excommunication.
I suppose it can be used against individuals and ordinary people, but I’ve only ever heard of governments being excommunicated. The consequences can be dire, on both sides. The excommunicated one (in this case the Minister of Justice was being specifically targeted) is denied the chance to earn merit, which means that his chances in the next life are substantially reduced, and his “luck” in this will quickly turn from good to bad.
The monks are taking a risk with upturning their bowls. The Cambodian government has sent monks to jail for it, and the Burmese military shot monks in 2007 when they refused to accept alms from the families of government officials, soldiers and senior members of the military junta.
In the absence of democracy, the monkhood represents the only organised opposition to government, and its ultimate sanction is excommunication.