If you are using a web form then the field should have the ability to be reset to blank by selecting the top item in the drop down (which is blank). This is just like when you test the form. If this is not the case then please let me know what Browser you are using and email me a link to your form.
If you are using PDF then the answer is a bit more complicated. If you do not save the form data after submit then all answers are cleared. If you have Acrobat Pro you can use the Clear Form tool (even after saving the form data). This will clear all form data. If you have Adobe Reader then you do not have the ability to clear the form data once you have saved the file.
I hope this helps.
Software Quality Engineer
Adobe Systems, Inc.