We can use existing WoT/"network" metrics to deal with that. 

 You can make it a paid relay only, and that alone makes a lot of difference. You should be able to moderate keywords in trending. In the future, we can also have a web of trust and proof-of-work like solutions to solve those problems. 

But right now, we need some kind of starting point. There are solutions to these problems but of course it will take some time.