behavior submit button

Hi,
I am new at wordpresss and trying to make a simple site with a contact form 7 at http://peb-wallonie.com/?page_id=24
When the checkbox with ‘the general terms’ is not checked the submit button is greyed out.
As my clientele are mainly elderly people they will maybe not understand why they can not submit.
I would like to change the behavior on hover over the submit button from a ‘forbidden icon’ to a text saying ‘you have to agree with the general terms’.
I would much appreciate if someone could help me with this.
Regards,
Philippe