Is the prize money from competitions that are not permissible in Islam lawful or unlawful, and is it permissible for someone who was ignorant of their unlawfulness to take it?

Money acquired from forbidden competitions is illicit money that must be disposed of by spending it on public utilities. As for someone who acquired it out of ignorance of its prohibition: some scholars permit him to benefit from it, but the more cautious approach is to dispose of it.