Change values in drop down


I am trying to create a form for signing up volunteers for an event. The user selects the event name and date and then select the volunteer position (such as greeter, ushrer, emcee, etc.) and submit. What I would like to do is show only those positions that no one has signed up for yet. For example, first person see a list of 5 positions and signs up as volunteer for greeter position. THe second person seeing this signup form will not see greeter as one of the options in volunteer position dropdown as someone already signed up for that position. How can I achieve this?