What would be a good way to take care of a rejected piercing scar ?

Not sure: What do you mean by a "rejected" scar? If by "piercing" you mean the scar is painful, it is possible that a nerve may be entrapped in the scar. This may require surgical excision of the scar and searching for the nerve end, and then burying that nerve end deep in the tissues.

See your doctor: Scars mature and can continue to improve for up to a year. They are sun-sensitive and may become hyperpigmented with sun exposure. The best way to treat a scar depends on the scar. Options include silicone gel, silicone sheeting, use of filler (for spot or crater-like scars), laser therapy, hydroquinone, dermabrasion, or kenalog (triamcinolone) injection, or surgical scar revision. See your plastic surgeon.