You should be able to add regular expressions to the word filter for email addresses in the configuration of the default spamblock plugin. (Which I never tried myself, but it says so.) You also need to activate the word filter by setting it to either moderate or block. I assume that's what you tried, but it's not working? Or are you using this with a different plugin and/or in a different config option?
@YL: Yes, I tried exactly what you thought in the default spamblock plugin but it didn't work. However, the solution provided by thh obviously worked. I will monitor this.
@thh: Thank you very much! That seemed to do the trick.