I have two good friends with this problem. One of them has a very severe case of Acne keloidalis nuchae and the wound is deep. He had to get a surgery to cure.

My second friend has the problem but it was still only skin deep. He said he applies Sulfur8 ointment on the area for four days to clear it, and that this treatment might need to be repeated whenever the symptoms start again.

Sulfur8 ointment is very easy to find in pharmacies. Here is a link with a picture of how it looks like:

https://www.amazon.com/Sulfur-Hair-Scalp-Conditioner-Ounce/dp/B00DN2727Q

I have also attached a picture with this post.