Brush the play dough with a stiff brush. A nylon bristle brush works well. You also can use the flat side of a knife to loosen the play dough. Vacuum up the loose pieces. Repeat several times to remove as much of the dried play dough as possible.
Wash the area with a little bit of cold water and a rag to remove the rest of the play dough. Do not get the carpet backing wet. Vacuum again when you are finished and repeat if necessary.
If there is a stain left behind, moisten a white rag or cotton ball with rubbing alcohol and blot at the play dough stain. Blot with cold soapy water made with mild dish detergent or liquid hand soap and water to remove stubborn play dough stains. Continue to blot or rub in one direction. University of Illinois warns that rubbing in circular motions can harm the texture of the carpet.
Mist the carpet with water and blot up the detergent or rubbing alcohol to rinse. Repeat this several times, but do not apply so much moisture that the backing gets wet.