If you get a warning message like 

Warning: exec() has been disabled for security reasons

The you are running a windows server and have the option to check submitters IP against a black list, while your system administrator has disabled exec()

There is not much you can do if you do not have full admin access to the server, apart from to turn off the check against the black list which you can do like this:

• Joomla Forms admin Console -> Select the Form
• Click Spam Controls
• Click IP Banning Tab
• Turn OFF the "Check Submitters IP Address Against Blacklist Databases"