Hi Maureen,
If I understand correctly, you do not want to completely remove the Activity Status dropdown for certain Users just certain options within the dropdown? I'll have to look more into if that's a possible option.
One thing that did come to mind you can try, (which we do on certain stock screens):
You can try using the Show/Hide Field/Object action that will check to see which Role the current active User viewing the screen is apart of. Then Show/Hide the correct dropdown for the User. Thus you will have more than one Activity Status dropdown and Show/Hide the correct one for the current User.
On the Users table there is a field called 'Roles' which holds which Roles the User is assigned to.
OnScreenLoad execute a Retrieve Value From Table for the Users table and match with of the current Active Users RecordID. Retrieve the Roles field for the User. Then in a condition statement you can do a IF RoleX THEN dropdownA ~ IF RoleY THEN ~ dropdownB - etc...
I hope that is clear and I have not tested on my end, give it a shot and let me know of the outcome and/or if you need to me to elaborate.
-- Mortaza