There was a huge blow up in my family at election time over this and two of my aunts are barely speaking to the third becuase the third aunt had enough of thier offensive emails and told them to cease and desist. Of course aunt #3 threw everything but the kitchen sink at them becuase she was so angry over years of "Christians don't support candidate X" and "God only want Candidtae X". So by the time aunt #3 blew up, she was ranting about how the other two thought she wasn't a good Christian, her life is happier, her kids more sucessful, she's made fewer life mistakes, she's more educated than the other two, etc. Trust me, its not worth it, its far easier to delete and go on with life.
If, however, you are like me and can't take your own good advice

, here's how I handled it:
1. Emails coming from my aunts- I would reply to all with a link to a snopes or urbanlegends.com that refute the basic facts in the email. I would add a commentary about how I love democracy and think its great to advocate for your point of view, but please do it truthfully. Lies about candidate x only weaken candidate y's position. That shut them up pretty quickly, since its rather embarassing for them. It also kept me from being a bad guy. Who can argue with a "check it out before you forward it on" request.
2. Emails coming from a frienemy- I tried tactic 1, no avail. I emailed him and told him to quit emailing political stuff, no dice. I finally told him if he didn't quit, there'd be retalitory tatics. When he emailed the next one, I promptly sent the DNC $50 on his behalf, then signed him up at moveon.org, dnc.org, barackobama.com, now.com, plannedparenthood.com, peta.org- every liberal think tank, money raiser, candidate in national state and local races, he was put on thier mailing lists. I donated to Obama in his name, to move on in his name, to the dnc. He is now a donor to those organizations and will NEVER get off thier lists. If you can't beat 'em, join 'em.
Not so proud of tactic 3, but the frienemy was expendible so I wasn't worried about loosing the relationship, and I have to admit, it was fun. Wouldn't recommend this route with your BIL, though.