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When Does CiviCRM Create a Contact Record for a User? When is a User Linked to an Existing Contact?http://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=14452376
note that a civicrm profile (and hence a contribution/membership/event registration) page has the ability to create a joomla user (check create CMS account in advanced settings for a profile)
I would use a contribution page + profile with J user account creation turned on.Re: the J login module, you have a couple options --1) modify the module template with at template override so the option is removed2) use an alternate login module (several give you the option of turning off the registration option)3) try the first plugin on this page: http://wiki.civicrm.org/confluence/display/CRMDOC22/Plugins+for+Joomla+(registration+restricted+to+contacts,+check+membership+expiration,+force+civi+registrationThe last option may be the best, but I haven't looked at it recently and don't remember if it will only let you redirect to a profile, or if you can redirect to a contrib page.There was discussion in the J world about adding an option to enable user registration, but turn off the reg link directly through the global config so that you separated the registration function from presentation. But I'm not sure it gained any traction.