Web Server Attack

Over the past hour I’ve been struggling with what seems like an attack on my VPS hosting service. Technical support have reported as follows:

It seems your server was being hit hard by the following IPs … They were creating so many apache processes your server was running out of RAM which was causing the services to fail.

I run almost exclusively WordPress websites (The IP’s originate in Romania … and one of the high-traffic websites on the host is a WordPress Romanian install). Is there something I can do in WordPress to prevent this? (search forums and plugins – and couldn’t find anything that seemed relevant to this).