How to remove your BetFair account
From their FAQ: If you wish to close your account you simply need to contact us via telephone or by the registered email address on your Betfair account requesting the account to be closed. We will then process the closure and confirm this to you.
For people wishing to call, see: https://www.betfair.com/aboutUs/Contact.Us/
Direct removal link: https://www.betfair.com/aboutUs/Contact.Us/
Helpful Trick: Removing online data
Publicly visible text/images often aren’t properly removed even when you succeed in removing an account. Try editing or removing them manually before removing the account itself. If you’re unsure what happens to your tracks: this can be found mostly in the Terms of Service/Privacy Policy; otherwise you can always contact BetFair and ask personally. By the way, removal requests don’t necessarily mean your data will actually be removed (e.g. due to legal obligations).
Helpful Trick: Remove third-party logins
If you signed up for BetFair using a third-party login such as Facebook or Twitter, you probably want to revoke all access from BetFair. Click here to find out how.