> If I am understanding this correctly, GDPR means that you must legally accept free riders, ie. you cannot deny service if they don't want it.

That is more or less correct, although some newspapers in Europe are taking the gamble on that and there might be grounds for a middle-ground decision and amendment of the GDPR down the line.

> who provide no value at all

Disagreed, those users still provide value - not only can you still advertise to them based on public information (and users put out a lot of it by liking/following certain content) but they still strengthen the network effect of the product.