The delete option in the real time threat analyzer will just mark the alert for deletion for the next time the purge utility is run.
If you want it to delete now you need to run the dbadmin.bat utility and do a database purge. The utility is under /App/bin in the install path.
You can also use the utility to clean up the oldest alerts.
The documentation that covers this is available on page 104 of https://kc.mcafee.com/corporate/index?page=content&id=PD23491
the link was created on the old support website. It was refering to the Administration Guide. You can find them in the new website when searching for "Administration guide" and choosing "Network Security Manager" as platform.
Here are the links to the current guides:
Administration guide for NSM 7.1:
Administration guide für NSM 7.5:
Administration guide for NSM 8.1: