What moral transgressions require us to take action?

It has always been my understanding that if someone initiates the use of force against someone we are morally obligated to act to mitigate that force--whether that be direct physical confrontation, destroying the aggressor's economic base, or otherwise ensuring that this person cannot attack innocent people further. However, are we obligated to take action against people who commit immoral acts but who do not initiate the use of force? For example, a shop owner can refuse service to black people. It's his property, after all. It's an immoral act; however, it's not initiating the use of force against anyone. Another, more troubling example is an institution with mixed moral and immoral premises--for example, Occupy Wall Street or the Tea Party. Should we feel obligated to act against these organizations? Or is inaction a rational option?