prohibited
The field under validation must be empty or not present.
prohibited_if:anotherfield,value,...
The field under validation must be empty or not present if the anotherfield field is equal to any value.
prohibited_unless:anotherfield,value,...
The field under validation must be empty or not present unless the anotherfield field is equal to any value.
prohibits:anotherfield,...
If the field under validation is present, no fields in anotherfield can be present, even if empty.