Not too much you can do. Spam blockers work to some extent, but the spammers get smarter and know how to get around them. You could try adding filters to your email filtering program, if you have one. Just removing mails that contain the words "Cialis" or "Viagra" may help, but then they get around those by misspelling the words.
So, call your congressman or parliamentary representative. Do not try to unsubscribe from these since that is really just a verification for them that your email is valid. You could also try reporting these guys to your ISP or their ISP's, but this only helps in a small percentage of cases.
As for where they got your address? Could be anywhere. They sell email lists, bots scan the net for addresses, some just send out to randomly generated addresses hoping that many will be real addresses.
Just keep pressing DELETE. :-D