Occasional stonewalling is not bad. However, doing this consistently during an argument or as a way to deal with your partner's communication can become detrimental to the relationship. So what's the harm? People who stonewall repeatedly refuse to respond and appear to have an invisible shield up. They might tune out by turning away, acting busy, or engaging in obsessive behaviors. They usually do this as a way to end the arguing, but at times this actually escalates the arguing. It is frustrating and even infuriating when dealing with a stonewaller. The point it is bad for the relationship is when you engage in stonewalling because it's the best way for you to resolve the argument, when this becomes the only resolution.
What to do instead: when you are reaching your boiling point, first STOP. Secondly, take a break from each other and do something that can calm you down (listening to music, taking a shower). Only return after at least 20 minutes or more, and find a way to acknowledge what your partner was feeling. Acknowledging your partner's feelings first can decrease tension such as defensiveness and contempt (which are the other horsemen), and encourage more open conversation.