I don't think you can prevent Puppet from reporting the file changed, but you could try declaring
show_diff => false
on the File resource by which it is managed. That will prevent Puppet from emitting the details of the change into its log, and it might also prevent those details from being included in the report sent back to the master (when that feature is enabled).
If by "reports" you just mean Puppet's log, then that should do the trick. If instead you mean it in the Puppet sense, and turning off show_diff doesn't work, and you're really worried about the credentials appearing in a file on the same server that provides the credentials in the first place, then perhaps you can implement a report processor on the master that will strip the credentials from reports.
Alternatively, you could manage the file via a custom package. Then Puppet would report when the package was updated, but the report (log or bona fide report) would not contain details about the changes.
John