The best way to go about this is to create a rule that allows those e-mails to automatically get deleted. Look for common words, sentence, items, or combinations, create a rule using them and set your preferences - send to delete box, delete automatically without going to the delete box. Play around with the rules at first till you get the hang of it as you don't want legimate e-mails getting deleted. I've had to do this in the past and it works pretty well once you get the hang of it.
The other thing I would recommend is only giving your e-mail address/es out to "trusted" parties, and informing your friends that you do not approve of them including you in their mass forwarding e-mails.
It might hurt their feelings for a minute, but it's your e-mail box that suffers long term