Answer by Peter Seale for Use -notlike to filter out multiple strings in PowerShell

$listOfUsernames = @("user1", "user2", "etc", "and so on")
Get-EventLog -LogName Security | 
    where { $_.Username -notmatch (
        '(' + [string]::Join(')|(', $listOfUsernames) + ')') }

It's a little crazy I'll grant you, and it fails to escape the usernames (in the unprobable case a username uses a Regex escape character like '\' or '(' ), but it works.

As "slipsec" mentioned above, use -notcontains if possible.