I think you misunderstood what was proposed. The OP wasn't asking to select multiple items, but to DESELECT all items in a single step. A sort of reset for the group.
I really hope I've missed something and this is possible, because I have a calendar in one prototype and I really don't want the pain of individually toggling the selected state of each day just to reset it.
The number of inelegant and brittle kluges necessary to persuade Axure to do basic things is becoming insurmountable -- especially given the number of excellent alternatives available.