People sending trackback pings from weblogs that don't have any link to your weblog seems to be a popular subject today. In my book, this ranks right up there with commenters who say "Nice post!" just to get their blog's URL on your comments page.
My policy is pretty much the same as what I've read on other weblogs today: the trackbacks are summarily deleted. In addition, I usually consider it about the same as trackback spam and therefore a death penalty offense - I also add the weblog's domain to MT-Blacklist, so I'll never see that weblog's URL in a trackback or comment again.