If you want to prevent form submits from public emails like Yahoo, Gmail or AOL you can set it up in ActiveDEMAND form editor.
Step 1: Open the form and edit the Email Address field by clicking the pencil icon:
Step2: In the EDIT FORM FIELD menu choose the Custom CSS Class and type "nofreeemail" :
Next time if someone uses a public email, there will be a message "Please use your company email address. Public email addresses (Gmail, Yahoo, and AOL) are not allowed":