Hi Joey,
You could make the field mandatory/required (see Object > Value palette). Depending on how you have set up your submit button it will not proceed with submission if field is null.
There is a new method called cancelAction, which can be used specifically to cancel the submission. However you need to set the target version to Acrobat/Reader v9.1 in the File > Form Properties > Defaults tab and the users will need to use this version of Acrobat/Reader.
Check out some of the email submission solutions here: http://assure.ly/eUR4wJ.
Hope that helps,
Niall