File -> Report Options -> Save Lock Report Design
You will be prompted to provide a password.
-Save the report.
-Now whenever the report is opened in CR designer, it will prompt for Password. Now it is secured.
The simple way to achieve your goal would be restrict user right for CR Designer. If the user has right to the designer, you may only have option by 3rd party software (it they exist at all) to actually LOCK your report for other to open
Editor, DotNetSpider MVM
Microsoft MVP 2014 [ASP.NET/IIS]