What can I call ‘this’?

I don’t know what they’re called, they make it difficult for skateboarders to grind. It damages benches, etc.

These are skate deterrents. They could both be described as anti-skate studs. They prevent skateboarders from “grinding” on street furniture, making them less likely to congregate in public spaces. Widespread now in major UK cities. The y are fairly uncontroversial but are sometimes discussed in the same context as “hostile” architecture, which is an interesting urban design topic. 

As a designer, and a skater, I’m on the edge about these. It’s like the middle bar on a public bench. It’s a middle finger to a specific group of people that they are not welcome. But I do get the use of them. I have seen the result achieved in a more aesthetically pleasing way. Check out this pic. Another option is to design the pavement or surrounding layout in such a way that they cannot get a ‘run’ built up towards the component.