I managed to solve this problem, complicated because of IWP.
1. Created a table for each user containing their specific pick list (fortunately, I knew in advance how many maximum users I would have.)
2. Created copy of the layout the drop down pick list field appears for each user
3. Created a script on each layout for each user that deletes the previously chosen picks
4. The next time that user logs in and selects new record, the user is taken to their specific layout, which now has the amended pick list with fewer options.
Lots of work but as a newbie, this was the only way I could think of making it work and it's tested by 30 of the users, so success it is.
I tried tons of other options with one huge script that did work in Filemaker Pro client but DID NOT work in IWP.
Hope this helps anyone else going forward.