puritan
n. countablen. someone who has very strict moral rules and believes that pleasure is often wrong or dangerous. You might use this to describe a person who thinks others should live more simply or strictly.
n. a person who adheres to a strict moral or religious code, often characterised by the rejection of luxury and physical pleasure. While historically referring to 16th and 17th-century Protestants, the modern sense describes any individual with rigorous, austere standards of conduct.
My uncle is a bit of a puritan when it comes to diet.
The film's director was accused of being a puritan for refusing to include any scenes of celebration or excess.
Probably from purity + -an, because they sought to purify Church of England worship of what they perceived as Roman Catholic aspects. The capitalised form preceded the lowercase form puritan.
When referring to the historical religious group, it is typically capitalised; in the general sense of a strict person, it is usually lowercase.