The safest approach is to start with the gentlest method and only move to stronger products if you really need to. Graffiti removal is mostly about matching the cleaner to the finish, not about scrubbing harder.
First, put on gloves and eye protection, and work in a well-ventilated area. Before you touch the graffiti, test any cleaner on a small hidden spot of the furniture. This is especially important on powder-coated metal, painted wood, anodized aluminum, and plastic. Wait a few minutes, then check whether the finish has softened, dulled, or changed color.
For fresh or light graffiti, begin with warm water and a few drops of mild dish soap. Use a soft cloth or a soft nylon brush, and let the soapy water sit for a minute or two. Wipe gently and rinse well. Many tags are easier to lift once the surface is wet.
If that does not work, try isopropyl alcohol on a cloth for powder-coated metal or tough plastic. Do not pour it directly onto the furniture, and do not let it pool. Wipe a small area, rinse with clean water, and dry with a soft cloth. A citrus-based adhesive remover can also help with sticker residue and some spray-paint marks, but check the label first and test it on the finish.
For stubborn graffiti, use a commercial graffiti remover that is labeled as safe for the specific surface. Apply it to a cloth rather than the furniture, use the shortest dwell time recommended, and rinse thoroughly afterward. Avoid acetone, lacquer thinner, wire brushes, steel wool, and high-pressure washing too close to the surface. Those can strip paint, scratch powder coating, or force water into joints.
A plastic scraper can help lift thick paint or sticker residue, but keep it almost flat and use very light pressure. If the furniture is painted wood, be extra careful: solvents can lift the paint along with the graffiti. In that case, a mild graffiti remover or a professional cleaning service may be the wiser choice.
After cleaning, rinse the whole area with clean water and dry it. If the finish looks dull, apply the recommended protective wax or sealant for that material. For public furniture, follow local rules for cleaning products and runoff, and keep chemicals away from plants and drains. Removing graffiti quickly usually makes the job easier and protects the finish.