Treating acne on the neck and jawline can be a totally different ball game when compared to combating facial acne. This is primarily because the skin on your neck reacts differently to normal treatment methods. The skin on your neck will likely be much more sensitive than that on your face, and therefore it is crucially important that you don’t do anything that will over-dry or risk damaging your skin.
As far as acne on the neck and jawline is concerned, a principal cause of this could potentially be lactose or dairy intolerance. There seems to be a strong link between jawline acne and dairy (especially cystic acne along the jawline) You should try cutting out lactose and dairy as much as possible for a while, and see if this has any impact on your acne; if so, then you need to consult a doctor.
For guys incorrect shaving technique can be a big contributing factor to acne on the neck. You need to exfoliate before shaving to soften up the hair, and ensure you use a clean, sterilized blade every time. I would suggest to avoid using an electric shaver as these are more irritating on the skin, and can also have a bad buildup of bacteria. Boil a cup of water and dip the razor blade in there for a few minutes, then apply some hand sanitizer gel and wash it off after 20 seconds. Incorrect shaving technique, or dirty/dull razor blades are very likely to cause a worsening of neck acne.
In terms of general jawline and neck acne treatment you need to be gentle, and you MUST avoid squeezing neck spots, as they are very prone to going cystic and leaving nasty scars or red marks. The secret is being careful and delicate with the skin on your neck, as it is very sensitive. Although facial acne can sometimes be hidden with careful squeezing of spots when a head has formed, pimples on your neck will not react in the same manner, and will actually take far longer to heal.
As far as over-the-counter products that help, you should try something with a low concentration of salicylic acid. Neutrogena makes an inexpensive and effective face wash with 2.5% SA and grapefruit extract which would be ideal for using once a day on severe neck acne- just don’t forget to moisturize as well. You should avoid using overly harsh products though, as the sensitive skin on your neck will punish you for this with even worse spots.