(A general apology for the OT-ness, but I think 
most everyone could benefit from this sort of 
knowledge ^_-)


Why adding junk works to confuse harvester bots 
(unofficial take):

Firstly, if you have a paid-for domain like I do, 
you have to add the junk to the HOST side of your 
address. "username@host.com" Otherwise, if you're 
using something like Geocities or your school 
account, adding junk to the username side works 
just as well. What happens is that the bot 
harvests your address by searching for some kind 
of pattern like above - 
something@something.something

Which makes it into a big spam list that is often 
sold. When the spammers try to send something to a 
junked address - e.g. username@host.JUNK.com - 
since the server "host.JUNK.com" does not exist, 
the spam will BOUNCE back to the spammer. 

Same goes with a junked username, but, it's still 
better to junk the host side. Now, assuming that 
the spammers are not relying on live people to 
filter junked addresses, your junked address 
should be discarded as just "invalid" and they'll 
never try to bother you again. ^_^ In theory.

Of course, the drawback of adding junk is that it 
has to be EXTREMELY obvious so that normal people 
who you want to contact you will remove the stuff 
before sending a message. And from my own 
experience on this forum ^_^' it isn't always 
obvious unless you say so in your message. Oh 
well...