Попробуйте в .htaccess прописать следующее
RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} acs [NC,OR]
RewriteCond %{HTTP_USER_AGENT} alav [NC,OR]
RewriteCond %{HTTP_USER_AGENT} alca [NC,OR]
RewriteCond %{HTTP_USER_AGENT} amoi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} audi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} aste [NC,OR]
RewriteCond %{HTTP_USER_AGENT} avan [NC,OR]
RewriteCond %{HTTP_USER_AGENT} benq [NC,OR]
RewriteCond %{HTTP_USER_AGENT} bird [NC,OR]
RewriteCond %{HTTP_USER_AGENT} blac [NC,OR]
RewriteCond %{HTTP_USER_AGENT} blaz [NC,OR]
RewriteCond %{HTTP_USER_AGENT} brew [NC,OR]
RewriteCond %{HTTP_USER_AGENT} cell [NC,OR]
RewriteCond %{HTTP_USER_AGENT} cldc [NC,OR]
RewriteCond %{HTTP_USER_AGENT} cmd- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} dang [NC,OR]
RewriteCond %{HTTP_USER_AGENT} doco [NC,OR]
RewriteCond %{HTTP_USER_AGENT} eric [NC,OR]
RewriteCond %{HTTP_USER_AGENT} hipt [NC,OR]
RewriteCond %{HTTP_USER_AGENT} inno [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ipaq [NC,OR]
RewriteCond %{HTTP_USER_AGENT} java [NC,OR]
RewriteCond %{HTTP_USER_AGENT} jigs [NC,OR]
RewriteCond %{HTTP_USER_AGENT} kddi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} keji [NC,OR]
RewriteCond %{HTTP_USER_AGENT} leno [NC,OR]
RewriteCond %{HTTP_USER_AGENT} lg-c [NC,OR]
RewriteCond %{HTTP_USER_AGENT} lg-d [NC,OR]
RewriteCond %{HTTP_USER_AGENT} lg-g [NC,OR]
RewriteCond %{HTTP_USER_AGENT} lge- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} maui [NC,OR]
RewriteCond %{HTTP_USER_AGENT} maxo [NC,OR]
RewriteCond %{HTTP_USER_AGENT} midp [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mits [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mmef [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mobi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mot- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} moto [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mwbp [NC,OR]
RewriteCond %{HTTP_USER_AGENT} nec- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} newt [NC,OR]
RewriteCond %{HTTP_USER_AGENT} noki [NC,OR]
RewriteCond %{HTTP_USER_AGENT} opwv [NC,OR]
RewriteCond %{HTTP_USER_AGENT} palm [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pana [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pant [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pdxg [NC,OR]
RewriteCond %{HTTP_USER_AGENT} phil [NC,OR]
RewriteCond %{HTTP_USER_AGENT} play [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pluc [NC,OR]
RewriteCond %{HTTP_USER_AGENT} port [NC,OR]
RewriteCond %{HTTP_USER_AGENT} prox [NC,OR]
RewriteCond %{HTTP_USER_AGENT} qtek [NC,OR]
RewriteCond %{HTTP_USER_AGENT} qwap [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sage [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sams [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sany [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sch- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sec- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} send [NC,OR]
RewriteCond %{HTTP_USER_AGENT} seri [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sgh- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} shar [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sie- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} siem [NC,OR]
RewriteCond %{HTTP_USER_AGENT} smal [NC,OR]
RewriteCond %{HTTP_USER_AGENT} smar [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sony [NC,OR]
RewriteCond %{HTTP_USER_AGENT} sph- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} symb [NC,OR]
RewriteCond %{HTTP_USER_AGENT} t-mo [NC,OR]
RewriteCond %{HTTP_USER_AGENT} teli [NC,OR]
RewriteCond %{HTTP_USER_AGENT} tim- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} tosh [NC,OR]
RewriteCond %{HTTP_USER_AGENT} tsm- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} upg1 [NC,OR]
RewriteCond %{HTTP_USER_AGENT} upsi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} vk-v [NC,OR]
RewriteCond %{HTTP_USER_AGENT} voda [NC,OR]
RewriteCond %{HTTP_USER_AGENT} w3cs [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wap- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wapa [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wapi [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wapp [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wapr [NC,OR]
RewriteCond %{HTTP_USER_AGENT} webc [NC,OR]
RewriteCond %{HTTP_USER_AGENT} winw [NC,OR]
RewriteCond %{HTTP_USER_AGENT} winw [NC,OR]
RewriteCond %{HTTP_USER_AGENT} xda [NC,OR]
RewriteCond %{HTTP_USER_AGENT} xda- [NC,OR]
RewriteCond %{HTTP_USER_AGENT} up.browser [NC,OR]
RewriteCond %{HTTP_USER_AGENT} up.link [NC,OR]
RewriteCond %{HTTP_USER_AGENT} windows.ce [NC,OR]
RewriteCond %{HTTP_USER_AGENT} iemobile [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mini [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mmp [NC,OR]
RewriteCond %{HTTP_USER_AGENT} symbian [NC,OR]
RewriteCond %{HTTP_USER_AGENT} midp [NC,OR]
RewriteCond %{HTTP_USER_AGENT} wap [NC,OR]
RewriteCond %{HTTP_USER_AGENT} phone [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pocket [NC,OR]
RewriteCond %{HTTP_USER_AGENT} mobile [NC,OR]
RewriteCond %{HTTP_USER_AGENT} pda [NC,OR]
RewriteCond %{HTTP_USER_AGENT} PPC [NC,OR]
RewriteCond %{HTTP_USER_AGENT} Series60 [NC,OR]
RewriteCond %{HTTP_USER_AGENT} Opera.Mini [NC]
RewriteCond %{HTTP_USER_AGENT} !windows.nt [NC]
RewriteCond %{HTTP_USER_AGENT} !bsd [NC]
RewriteCond %{HTTP_USER_AGENT} !x11 [NC]
RewriteCond %{HTTP_USER_AGENT} !unix [NC]
RewriteCond %{HTTP_USER_AGENT} !macos [NC]
RewriteCond %{HTTP_USER_AGENT} !macintosh [NC]
RewriteCond %{HTTP_USER_AGENT} !playstation [NC]
RewriteCond %{HTTP_USER_AGENT} !google [NC]
RewriteCond %{HTTP_USER_AGENT} !yandex [NC]
RewriteCond %{HTTP_USER_AGENT} !bot [NC]
RewriteCond %{HTTP_USER_AGENT} !libwww [NC]
RewriteCond %{HTTP_USER_AGENT} !msn [NC]
RewriteCond %{HTTP_USER_AGENT} !america [NC]
RewriteCond %{HTTP_USER_AGENT} !avant [NC]
RewriteCond %{HTTP_USER_AGENT} !download [NC]
RewriteCond %{HTTP_USER_AGENT} !fdm [NC]
RewriteCond %{HTTP_USER_AGENT} !maui [NC]
RewriteCond %{HTTP_USER_AGENT} !webmoney [NC]
RewriteCond %{HTTP_USER_AGENT} !windows-media-player [NC]
RewriteCond %{QUERY_STRING} !wpc_nr [NC]
RewriteRule ^(.*)$ site.ru/ [L,R=302]
Не помню, откуда список юзерагентов взял. но использовал в свое время именно это.
