In the spirit of a DVCS (as in "Distributed"), you don't cancel something you have published:
Pull requests are essentially patches you have send (normally by email, here by GitHub webapp), and you wouldn't cancel an email either ;)
But since the GitHub Pull Request system also includes a discussion section, that would be there that you could voice your concern to the recipient of those changes, asking him/her to disregards 29 of your 30 commits.
Finally, remember:
- a/ you have a preview section when making a pull request, allowing you to see the number of commits about to be included in it, and to review their diff.
- b/ it is preferable to rebase the work you want to publish as pull request on top of the remote branch which will receive said work. Then you can make a pull request which could be safely applied in a fast forward manner by the recipient.
That being said, since January 2011 ("Refreshed Pull Request Discussions"), and mentioned in the answer above, you can close a pull request in the comments.
Look for that "Comment and Close" button at the bottom of the discussion page:
