After a discussion at nowiki it became clear that Sarah Michelle Gellar (Q180665 was given a political affiliation with the Republican Party (Q29468) even if she explicitly said she had no such affiliation. The thing is that such statements can be made without any source.
A similar case is affiliation with the political party Nasjonal Samling. Such statements should not be allowed without references.
I propose that sources can be enforced for a property, both in a property, then enforcing sources everywhere for this specific property, and in a specific item, then enforcing sources for the property in that item only.