Add a ValuesNotAllowed / ValuesDisallowed attribute to the field object on a page for enums and options. This would work alongside the ValueAllowed attribute and the ValueAllowed would take presidence for backwards compatibility.

There are two good reasons to add this.

1. Easier maintenance. Every time I use the ValuesAllowed attribute I find myself having to add every value except for one or two. I have yet to find a scenario where inclusive logic makes sense. Only having to add one value of a large enum list would make life so much sweeter.
2. Better extension support for field option/enums. ValuesNotAllowed would only affect the base apps enum values. Extensions can't extended the ValuesAllowed attribute so it makes me often avoid using this very useful attribute.