We would like to would like to take out access from Project Owner group to add any users, in contract workspace. So that they can't 'add more users' as project owner. How can we do that or is this something can be done by Ariba support only.
To restrict the project owner from adding/removing users in the Contract Workspace project, you can customize the Contract Workspace template as below:
Contract Workspace template> Edit Teams> click on Project Owner and select "No" as an answer to the question "Can owner edit this Project Group". Once the template is published with the change, the project owner will no longer be able to add/remove users in the Contract Workspace project.