Scrape off large pieces of plaster using a 6-inch drywall knife. Use the knife at a low angle to the wall to avoid gouging, and gently scrape off the big chunks.
Sand the area with a drywall sanding sponge until all traces of the plaster are removed. Lay the sponge flat against the wall, and move it over the affected area using smooth, even strokes.
Wipe the wall with a damp sponge to remove traces of drywall dust created by the sanding process.
Paint the area of the wall using a 2-inch paint brush and touch-up paint that matches the rest of the wall. Apply a second coat once the first has dried.