I have been receiving large volumes of spam to random, numerical addresses at my domain. For example 48321819.1928382@, and so on. It’s not practical to block a single address. The spammers make up new addresses at random.
I wanted to block addresses using wildcards. For example, anything that starts with a 4 and ends with @domain.com. I use postfix, so I did some research. I didn’t find much, so I asked on the #postfix FreeNode channel. The very helpful sysmonk introduced me to regex tables in postfix.
So, I have now managed to block a whole bunch of addresses using a few simple regex patterns. The amount of spam I receive has dropped by almost 90% already. Yay! 
Callum,
I find it very odd to hear that you have loads of spam. Personally, I use the ZEN blacklist (http://www.spamhaus.org/zen/), together with SpamAssassin, and Greylisting (http://postgrey.schweikert.ch/)
Especially the latter is a project you should have a look at!
Cheers!
Diederik