How to use rules to hide some options in a select field?

hello.. I want hide an option in a select field if (..) ..

e.g. I have a field_food ckk field on a recepies content type (it's almost dinner time here...) with:
a|potatoes
b|pasta
c|tomatoes
d|cake

and in the add recepies node form I want hide the cake option of the field_food.

I managed to get the form event, but I don't know how to alter (hide) a single option of a form element..

Any help?