Search abuse

My website is getting searches at very high rate, after checking access_log i found that there are thousands of request using this method

example.com/?s=query

to overload my website, i am facing this attack from last month, these searches are performed by thousands of ip address that are changed if i block them, hundreds of irrelevant querries, my server is overloaded loaded, i am using nginx as reverse-proxy, somebody please help me with this.