Start mild and work up. First figure out what the furniture is made of—powder-coated steel, aluminum, wood, concrete, plastic, or glass—because the wrong solvent can dull paint or soften plastic. For stickers and adhesive, warm the sticker with a hair dryer or heat gun on low, peel it slowly with a plastic scraper, then clean the glue with warm soapy water. If residue remains, try isopropyl alcohol on metal or glass, or a citrus adhesive remover on most painted surfaces. On wood, use the least aggressive route: a little cooking oil or citrus cleaner, then degrease with mild soap; avoid soaking. On concrete, a stiff nylon brush plus a poultice or graffiti remover may be needed, but test first.
For spray-paint graffiti, act quickly and use a product matched to the surface. On metal, denatured alcohol or a gel graffiti remover can lift fresh paint; use a non-scratch pad, not steel wool, if the finish is intact. On painted furniture, test a hidden spot and avoid acetone, lacquer thinner, or harsh wire brushes unless you plan to repaint. Pressure washing can help on concrete but keep the nozzle moving and use low pressure so you don’t etch the surface or blast paint off nearby fixtures. Always wear gloves and eye protection, work with ventilation, and keep runoff away from storm drains. Test in an inconspicuous spot, use the mildest method first, and finish with a soap-and-water rinse. If the piece is vandalized repeatedly, a sacrificial anti-graffiti coating can make future cleanup much easier.