Change Filter Form Element Items Activity Phrase

Is there a way to change the Phrase of a filter or even remove a filter from the Drop down List?

I have 2 drop down list on a form, one for Suburb and another for Postcode. When the user selects a Postcode I have a change event which sets a filter on the Suburb drop down. It works the first time I change the postcode and see a filtered list of Suburbs yet when I change the postcode a second time my Suburb list is empty, it's like its adding to the filter and not replacing it. I can see the event fire and activity run to set the filter yet the results are false the second time.

Thanks.