Is it possible to grant read only access to the event logs but prevent review of the ASM policies at the same time?

This is not possible. If you want logs-only access the best practice is to configure the ASM Logging Profile to send the ASM logs off-box to a remote logging server (e.g. Splunk). The just let your users access the ASM logs form within Splunk (or any other logging server whatever you have deployed in your company)