'a spell to bring on anorexia?'
anorexia is an eating disorder, you're too lazy to just get low self esteem and stop eating? trust me, as a person who has had family members and myself have eating disorders, it's not fun. people with eating disorders [most] know they should eat, but once they do, they feel guilty.
you could look into illusion spells, but it won't effect everyone. try a simple diet spell, coupled with a new diet [not a diet like weight watchers, i mean diet as in what you eat] just adding another 15 minutes of exorcize and slowly changing your eating habits will help you.
while i don't know you, try eating more chicken and fish [like salmon] over red meat [though you need some red meat, one serving a week imo] eat several small meals throughout the day, and if you choose to eat 3 meals, have your largest in the afternoon and your smallest in the evening, since you can't burn it off 3 hours before bed. you should also fill your plat with mostly vegetables. and while eliminating all junk food over night is impossible, cut back gradually.
it's no quick fix, but there you go.