A UDP, or Usenet Death Penalty, is a means by which site administrators and others around the world attempt to enforce the cooperative nature of usenet on an uncooperative member of that community. It is accomplished, in most cases, by what is known as an "active" UDP. An Active UDP is one in which every message posted to usenet by the offending site is canceled or failed to be propagated. This is done regardless of content, posting author, or whether or not it would normally fall under the spam cancel thresholds. This simply cuts that site, and all of its users, off from being heard by the rest of usenet. A UDP can also be focused at a narrower range of targets as well, such as "everything out of sprynet in alt.sex.*", or "Everything from user "email@example.com", or "everything out of UUNET from a particular dialup pool".
Taken from http://www.stopspam.org/usenet/faqs/udp.html