Almost every role in this atlas can be entered and left. A priest can be laicised, a bhikkhu can disrobe, a pastor can be voted out, an imam can be dismissed by a mosque committee. The kohen cannot do any of it.
Priestly status in Judaism is a fact about ancestry. It is not conferred, so there is no conferring authority to reverse it. A kohen who abandons observance entirely is still a kohen; his sons are kohanim; his obligations and restrictions remain in force whether or not he keeps them. The law does recognise a chalal, a man of priestly descent disqualified from priestly duties by his father's forbidden marriage, but that is a defect in the line, established at birth, not a punishment anyone administers.
The practical edge of this shows in modern hard cases. A kohen who wishes to marry a divorcee or a convert is refused by the Israeli rabbinate, which controls Jewish marriage in Israel, and the couple's usual recourse is to marry abroad and register the civil marriage on return. Men have sought rabbinic rulings casting doubt on their own priestly descent in order to escape the restriction, and the responsa literature on this is substantial and not sympathetic.
Reform Judaism sets the priestly distinctions aside, on the position that the Temple cult is not to be restored and its class structure need not be maintained. That is a decision about whether the category still means anything, not a mechanism for leaving it.