How can I set an RTC repository read-only while keeping some SCM functions active?

We've moved our users to a new repository, but left the source code history on the old repo.  We want to lock down the old repository to prevent updates, while still allowing users read access to everything (that they previously had access too)

I thought we could use the JazzGuests repository permission group, instead of JazzUsers, but this also prevents users from creating personal workspaces. 

Short of stripping all permissions from each project/team area, is there any way to accomplish what I'm trying to do?

Thanks