Hi Ben,
Deny all the edit rights on that folder where you have placed that universe by using granular rights and also provide view on demand rights to that group on this folder.
This way they would not be able to create any new report based on this universe, even would not be able to edit the queries of the reports but they would be able to refresh the reports which are based on this universe.
To make it more simple, create one custom access level and deny all these "edit" rights. Perhaps you would like to name it as "Restricted Universe Access Level" 
. This access level will save your time for assigning granular rights every time you want to make such changes.
I hope this helps . 
~Swapnil