I've spent planty of time on Sigma Pro without much success, however it does work and there are people using it, so I plan to give it another (4th) big effort soon. The developer often has bursts of activity, but has been quiet lately.

I've checked the forum there for queries that might relate to your issue and will paste a couple of his responses in - note that these are just snippets, not related to each other,  and may not specifically fit your issue:

Go back and check the Teams Can See All Items (This controls the View list in the task list.  Assigned, Relevant, Creator, etc) option the Security seciton.  Then check Hide Hidden Fields from Teams in User Interactions.

^This was my initial setup. I don't know why this didn't work.

You can only show them as disabled or not at all. One option is to set the visible property of the control to False in the ASCX files, but this will effect all projects. You could also use the Help Desk add-on module as an alternative UI.

I have not yet investigated HelpDesk as an option. SigmaPro has been great otherwise and has a lot of functionality that I need. I've put so much time into it that it seems I can/should be able to figure this last piece out.

Yes, log-in as host and then open an issue. You will see a checkbox next to each field in the issue, these are used to set visibility for users. By default, all un-checked fields will be visible but disabled. You can change this to hide them as well by using the " Show Hidden Fields " option in Edit SigmaPro/User Interactions.

Right, I've tried this. I am using a full access acount to edit, and then checking it with a user created with a role as a team member. This user still can modify any field in the task.