Dip a wooden tongue depressor into a jar of stain-remover paste. Saturate the paint with a thick layer of a paste designed for tough stains. Allow the paste to saturate the dried paint for at least 20 minutes.
Rub a lint-free cloth against the dried paint. The paint will slowly scrub off the canvas ceiling and onto your cloth. Fold and turn your cloth as you work, so that you're not redistributing the stain onto the canvas.
Fill a spray bottle with cold water and spray the canvas to rinse it clean. Allow it to air-dry.
Pour 1/2 cup ammonia into a large jar. Add 2 cups cold water. Replace the lid of the jar and shake well.
Dip a lint-free cloth into the mixture and scrub at the layers of dried paint. Re-dip your cloth into the solution of water as necessary.
Fill a spray bottle with cold water once you've removed all the paint from the canvas ceiling. Spray the ceiling with cold water to rinse away the ammonia residue. Allow it to air-dry.