How to sort/delete comments by a particular user?
Using Views Bulk Operations, you can create a view for all the comments written by a user, which allows you to delete all the comments written by a user without the limitation of deleting them in groups of 50 at time. When you click on the button to select the comments shown in the pager's page, you are asked if you want to select all the comments; in that way you can delete all the comments with a single click. As the view can be set to use batch operations, it is able to delete a great number of comments without any timeout.

You can create a view that is shown in the user profile page (as done on Drupal.org), or you can create a view that replaces the page shown in admin/content/comment. Being a view, you can filter the comments listed using different criteria, such as the content type of the node for which the comments have been created, the author of the comment, the author of the node to which the comment is attached, etc.

Views Bulk Operation doesn't come with a custom view to show the comments made from a user; it comes with a view that can be used to replace the page shown at admin/content/node. Using that view as example, you can easily build the view you need for the comments.