Spammers: a lot of queries to non existing pages

I have big problem: a lot of queries per second to non existing pages like www.site.com/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri '/' uri ' and so on.

My site bandwidth is up 15-20 times up to 20GB per day :(
my server is often downtime...

Which module could fight to these types of attacks?
Please help!

Thanks for any answer