You can delete you Causes account at anytime. Leaving Causes will mean that you won't get any campaign updates over email and your Causes profile and all associated data will be removed. To delete your account, go to your privacy settings and select Delete my Causes account.
To uninstall the Causes application from your Facebook account, access your Facebook Account Settings and remove Causes from the App Settings. Removing the application will mean that you won't be able to share activity on Facebook or invite Facebook friends to support a campaign.