I want to post my own vision:
RewriteCond %{REQUEST_URI} ^/$
RewriteCond %{QUERY_STRING} ^author=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?