Form.select: always show :prompt value?

Hi all

Normally I find it very practical that the :prompt value is not shown
anymore as soon as the user has selected a value from the option list
and then submits the form and it is re-displayed because of an error or
so.

But there are cases that I want it to display in any case, so is there
a way to display it although a value has been selected before?

Thanks for help
Josh