This 3 line piece of code has saved my server TONS of cpu cycles (my load went down from an average of 5-6 to 1!):

//spammers
if (strpos($_SERVER['HTTP_USER_AGENT'], 'MJ12bot/v1.0.8') !== false) {
	exit();
}

Unfortunately, legit users of the MJbot will suffer ... but such is life.