There are 2 ways to lock the Project Server Site for any changes.
Option 1: Include DisableWebDesignFeatures=”wdfopensite” property in site definition [Refer to Project Server: How to edit PWA site]
Option 2: Configure the permissions in Site Settings
Step 1: Navigate to Project Web Access -> Site Settings. Click Advanced permissions
Step 2: On Advanced permissions screen, click Settings -> Permission Levels
Step 3: Listed on this screen will be 8 default security groups
Step 4: Now, select the group for which you want to lock down the a) List permissions and b) Site permissions. Uncheck the Manage List check box.
Next, uncheck the options for ‘Add and Customize Pages’, ‘Apply Themes and Borders’ and ‘Apply Style Sheets’.
Once this is done, the selected group (i.e. users) will not be able to make any edits to site.