Simple way to "disable" enum menu items or disallow them?

[eduo]

Hi.

I'm looking for a way to disable the "default" menu item in a required field where the default value can't be pre-selected. My goal is to have the first item either greyed out or to have it throw a validation error if selected.

I'm trying to do this directly in the schema, but it doesn't seem to be possible.

[schmunk42]

Could you provide a simple example, please.