Is taking money from a brother who spends it on unlawful things considered theft?

It is not permissible to take someone else's money without a just cause, even if that person is a disbeliever, so how much more so if they are a disobedient Muslim. Aggression against their property is not justified just because they might spend that money on their forbidden desires. Spending money on sinful acts is an act of foolishness, and a fool should have their property interdicted. However, the matter of interdiction is to be referred to the religious judiciary and is not entrusted to individuals.