The only way in the current version is to click 'Edit' on the user and then click 'Cancel'. From what we are working on for the 3.0 release though you will also be able to delete the user from the manage users page and (more importantly) be able to optionally fire off events. Just like the other evens for new registrations, registration updates, and both new and updates, there will be another option to fire off the event for membership cancellations. For example you can send an email after the user cancels or execute a SQL query etc... The role event will not really apply since the user will be deleted but you get the picture.

Also, right now if you delete the user in DNN there isn't really anything harmful by doing this as far as Dynamic Registration is concerned. There is a query you can run (or schedule once in a while) that will delete from the response table for users that have been deleted. This table takes up very minimal resources/space though, we plan on adding some maintenance features in the future that will allow you to delete this from the admin but that still in the works.

Ill try and post an update to the upcoming enhancements in the 3.0 release. There are many, and future plans after this next release for even more AJAX components and field types etc... We are spending a great deal of time focusing the 3.0 release though on improving the back-end administration and also some new enhancements regarding events/question events. Not ready to set the release date in stone yet but its coming up...

-Chad