I've used Dyn Email gateway. It's $49 a year per domain and once you pointed your MX records there and set your servers to only accept mail from Dyn you'll see a pretty steep reduction in spam but you'll also see a significant drop in bandwidth used since Dyn only deliver email to the address you include the in the filter. You can also do the same thing for free with MX Guard Dog

Try enabling a spam filter such as SpamAssassin.