From the toolbar, click the menu item "Safari" then click on "Preferences".
The Safari preferences dialog box will open up. Make sure you are on the "Security" tab and click on "Show Cookies".
The cookies dialog box will open up. This will give you the option to remove cookies 1 by 1. Type in the poker room that you wish to join (in this example we have chosen Full Tilt). You will see all the cookies from the Full Tilt domain appear. Highlight each of these and click on "Remove Cookie".
The appropriate cookies have now been deleted. You can now close the dialog boxes and continue with the registration!