Currently you can exclude specific day from the date picker (e.g. holidays),
but what you're asking would require #4557 to be implemented (see link
below). I've added a +1 from you on that one. In the meantime you can do
this through a formula. This will still allow users to select a date in the
future, i.e. dates in the future won't be grayed out in the date picker, but
as soon as the date is selected, the field will be marked as invalid.