Im not sure if its easily possible, but maybe if the option for 'multiple options's is checked on an exposed form field. It can be output with checkboxes.

I have an example a little down this page.
http://www.ryancramer.com/journal/entries/select_multiple/

Will mean the customer/client doesnt have to hold down ctrl to select multiple options. And if they have a few selected, and dont have ctrl down they loose all their previous options. So this will also prevent this from occuring.