Scrape up any excess banana using a spoon. Remove as much banana from the carpet as possible.
Prepare a detergent solution for dissolving the lingering banana residue. Combine 2 cups of warm water and 1 tbsp. of nonbleach dishwashing liquid in a container.
Dip a white rag in the nonbleach detergent solution, and blot the remaining banana stain, working from the outer edges inward. Remove as much banana residue as possible.
Moisten another rag with cold water, and blot the soapy carpet to rinse out the detergent.
Inspect the carpet for any remaining banana. If any stain remains, pour 2 cups of cold water into another container, and stir in 1 tbsp. of liquid ammonia.
Dip another rag in the ammonia solution. Blot the remaining banana residue, working from the outer edges inward. Remove the remaining banana stain.
Moisten another rag with cold water, and blot the carpet until all the ammonia is rinsed out.
Dab the carpet with a towel, and thoroughly dry the carpet.