How to block certain usernames hitting Freeradius server

[Burn Zero]

Hi,

We have setup Freeradius latest version to help users authenticate,
authorize to 802.x WIFI. While analyzing logs, we found that certain
user names ( with random alphabets ) that are trying to authenticate
every certain minutes. Those are just invalid usernames some people
have configured in their phone/tablet/system. They won't even get
authentication success since those are anyways invalid usernames.

What I am trying to achieve is to prevent these usernames from hitting
Freeradius servers ( do username, group check in Active Directory)  so
that when those invalid usernames comes to Freeradius it would be
filtered and no longer go inside tunnels and then for username check
in Active directory.

Thank you.

-
BurnZero