Hi:
I've attached a simple sample form -- easier than trying to explain it! Right click on the link in the heading of the post and select save as. Then open the form in design mode and preview. Basically, what I've done is on the filter for the drop down list, I've used the double eval trick described here to create a concatenated list of the selected items, then filtered for items that are not in that list. This won't work for every scenario -- I think if your list had 'ape' and 'grape' in it, for instance, and 'grape' had already been selected, 'ape' would be filtered out, but for a list with distinctly different items, it should work.