I want to post my own vision:

    RewriteCond %{REQUEST_URI} ^/$
    RewriteCond %{QUERY_STRING} author=(\%|\+|\d) [NC]
    RewriteRule (.*) $1? [L]

First line detects only home page. I'll explain why. This "user-enumeration" feature works only on home page, so no need to rewrite all urls. 

Next we look for `author=` query string. It's obvious.

Finally, we just show **original** page without any blocks, redirects (301, 302) or bans (403). Shouldn't it act like a page with any other useless parameter?