You cannot remove roles from projects, a role always exists across all projects.
You might want to take a look at admin -> project roles, and edit the defaults, so that when you create a new project, no-one gets into the project roles automatically.
If you need to delete the default groups associated with roles that are generated when creating a Project, you must access each role from System > Project Roles > Search for the role > Manage Default Members, from that option you delete the group associated with the selected role.
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