Don't throw them in the bin as I was about to, put them to good use.
Thanks to some fine advice from Mumsnetters on another thread, it appears that companies have to pay for material sent to them via Freepost.
UKIP has a freepost address
Send your leaflets back to UKIP and get them to pay for the privilege. As an added bonus, they pay by weight. So if you think Nutty Nige would be interested in an outdated Argos catalogue, Phone directory from Skegness or anything heavy (but not dangerous) you care to think of, slip that in the package too. It will be a nice gift for him.
It won't bring UKIP to their knees, but it wil cost them a few quid and it won't half make you feel better.