How 2 stop saving to hard drive if mandatory fields are not filled in?

I have created a form that people will recieve via e-mail (in livecycle when i choose to distribute form I choose save and send later), they will then fill in, save to their hard drive and then email back (i am not using email buttons on the forms themselves due to complications with our email client).

I have some mandatory fields, and I have built logic into the form so that if they don't enter data in required fields a message pops up. However, some people are still not filling in all fields and they are still able to save the form to their hard drive whem mandatory fields have been ignored.

How do I build logic into the form to dissallow saves if all mandatory fields are not filled in?

Any help greatly appreciated on this one :)