The short answer is to make the furniture less rewarding to skate and more useful as furniture, without turning the plaza into a hostile obstacle course. I would start with placement. Long, isolated ledges and smooth benches invite skating because they offer a clean run-up and a satisfying edge. Group seating with planters, armrests, tables, or nearby landscaping so it no longer reads as a single skateable object. Break up straight lines and open stretches where skaters can build speed and land tricks.
Design matters more than signs. Choose benches with segmented seats, curved profiles, armrests, or slightly angled surfaces. Rounded or textured edges are harder to grind than sharp metal lips. A subtle texture on the seat or edge can also make the furniture less appealing for sliding while still being comfortable to use. If the furniture is already installed, add removable anti-skate clips, raised dividers, or planters at the ends of benches. These should be safe, visible, and maintained; avoid spikes, grease, or anything designed to injure someone.
Think about the whole space, not just the bench. Bollards, low planting, bike racks, and well-placed tables can interrupt skate lines and create natural barriers. Keep pedestrian routes clear and sightlines open, because furniture that feels crowded or awkward is less likely to be used as a skate spot. Good lighting and regular maintenance also help: loose parts, chipped edges, and worn surfaces make furniture look abandoned and easier to misuse.
Signage can help, but it works best when it explains the rule calmly and points to a better place to skate. If skating is a real local demand, the most sustainable fix is often to provide a designated skate area nearby. Talk to local skaters, maintenance crews, and nearby businesses before adding deterrents, because some fixes create new hazards or simply move the problem down the street. The goal is not to punish skaters; it is to keep public furniture safe, comfortable, and available for everyone.