git - How to cancel a pull request on github?

GitHub now supports closing a pull request

Basically, you need to do the following steps:

1. Visit the pull request page
2. Click on the pull request
3. Click the "close pull request" button

Example (button on the very bottom):

This way the pull request gets closed (and ignored), without merging it.

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:

If you sent a pull request on a repository where you don't have the rights to close it, you can delete the branch from where the pull request originated. That will cancel the pull request.

Go to conversation tab then come down there is one "close pull request" button is there use that button to close pull request, Take ref of attached image

If you have opened PR on gitHub, but it is not merged yet and you want to get rid off it, not just close, do this simple command:

git push origin --delete <your branch name of your PR>