I read a post this morning from Naren where he talks about deleting workitems. As he states this is a topic/request that repeatedly comes up in forums, blogs and training sessions and I have lost track of the number of times I have been asked about deleting workitems myself. While there is no technical way to delete work items BY DESIGN, there are a few things you can do to achieve a similar outcome.
Naren makes mention of a solution that Richard Berg offered whereby you simply Deny permission to everyone on the workitem. While this will incur the very small cost of disk space over time, it ensures the workitems don’t appear in any queries or reports. Worth a quick read!