Hi Tegan,
If you go into the Securities application and switch the Status of each of these users from Active to Inactive, then in the Assign To dropdown they should sort to the bottom with the other inactive users.
To remove them from the dropdown completely, remove their user groups in the Securities application.
Cheers,
Rachel
I phrased that badly; to remove a former user from the dropdown, remove the user from the user groups he/she is assigned to in Securities.
Thanks so much Rachel - this is perfect!!!