Block masters to add the members in individual project

I am using GitLab 10.3.5. As per my workflow, I enabled all the users as the developer for all in group and added the member as master only who need the merge permission in the separate project. By this access scheme, everyone can access all project, but only project members (masters) only can able to merge the merge request.

Here my problem is Master can able to add the member to the project. This will destroy my workflow. I want to disable this feature. I tried by comment the add_user method in the member.rb but this idea was not helped.

I want to disable this only for master, I mean only owner can add the members in the projects. How can I do?