This can’t be done using a multiple-select field, but it could be done using a link to another table.
Create a new table with records representing the various options you want. Because your multiple-select field is named “Units”, I’ll use that same name for this new table. Turn your {Units}
field into a link to the [Units]
table. In the [Units]
table, add a view named “Not Selected”, and add a filter to that view to only show records where the incoming link field from your main table is empty.
Back in your main table, edit the {Units}
link field properties to only allow records to be chosen from that “Not Selected” view. Now as you create links in the main table, each linked record from “Units” can only be linked once.