Summarize:

Some people only wash their hair every few days, but if you suffer from frequent outbreaks of scalp pimples, that may not be enough. Instead, try to shampoo your hair every day. This may help reduce the amount of oils in your hair, which can reduce the chances of having a pimple outbreak along the scalp.  You may even want to use a clarifying shampoo or alternate a clarifying shampoo with your regular shampoo. Pimples on the scalp are often caused by a buildup of hair products, dead skin cells, and oils, and a clarifying shampoo will be more effective at removing these. Try avoiding conditioner to see if that improves your scalp as well. Conditioners help moisturize hair, which may cause too much oil/grease to be trapped against the scalp. If you find yourself experiencing frequent scalp pimples and you wash your hair on a daily basis, the problem may be stemming from something you put into your hair. Try to avoid using hairstyling products and see if that clears up your scalp. Once you've identified the cause, you can try experimenting with different types of hairstyling products to see if the ingredients in those products are compatible with your skin.  Try using products that are water-based, or look for products labeled as noncomedogenic, meaning they're less likely to clog your pores and cause an acne outbreak.  Avoid putting hair products too close to the hairline. It's okay to use hair gel or pomade, for example, but try to apply it only to the main strands of hair without letting it touch the scalp or hairline. Some people prone to scalp pimples who wear baseball caps or sports gear (like a helmet) have an increased prevalence of heat/friction/pressure acne, sometimes called acne mechanica. If you believe that wearing a hat or helmet might be causing your scalp acne, try letting your scalp breathe more often. Or, if you must wear protective covering on your head, make sure you wear an absorbent headband or hair cover underneath your helmet. Showering immediately after removing your hat/helmet and using shampoo may also reduce the incidence of scalp pimples. Brushing or combing your hair helps remove dead skin cells and breaks apart hair strands that have been joined by your scalp's natural oils. This can help prevent acne by both removing the skin cells that may have clogged your pores and separating the strands of hair that may have otherwise held oils trapped against your scalp. If you're prone to scalp pimples, cutting your hair to a more manageable length and thickness may help reduce outbreaks. Having shorter and/or thinner hair can help decrease how much hair is holding oil, dirt, and bacteria against your pores.
Use shampoo daily. Avoid known irritants. Let your scalp breathe. Brush/comb your hair daily. Consider cutting your hair.