Hi James,
While there is a good way that Noel suggested one other way is to ensure that even if the changes happen in object formatting,formula code,report-SQL etc, they are not 'Saved' with the report.
A report can be 'password' protected to prevent a 'design-level' change.
This feature is available under Report Options >> Save Lock Report Design
It will prompt to key-in a password. Once this is done it will prompt for a password whenever a user wants to 'Save' the report after making any changes.
Another way would be to use the .rptr format that is supported since Crystal Reports 2011 onwards. A .rptr report can be viewed from any viewer and users wont be able to modify the report design.
Also you might think upon using a exported pdf file for report data distribution instead of a .rpt file.
Thanks,
Prathamesh