Stop multiple web requests (web security)

Since few hours, I'm having multiple requests coming from various IP's to our website occuring every second(maybe 4 or 5 requests per second). The website's usual traffic is about 3 to 5 requests per minute. The requests are very random, for example:

/gtalczp/197zbcylgxpoaj-26228e-dtmlnaibx/
/109/jxwhezsivr/10445_xwvpfdyzhea.cgi
/nouyaku.html
/index.php/43e3133-pmuwbfgoedakvxs/
/keyword_list/s_index=L

The site's indexing in google is now all in japanese characters and messed up.

I have tried blocking IP's(via .htaccess) that make all these random requests, but every time a new IP is making a new request. How can I stop all of these requests? Can I use an .htaccess rule that allows only the links that are available in the site?

EDIT: Our site is running Wordpress latest version, with custom built features.