Comma Separated List in VBO
The textbow must have a size so you wont be able to make more than what your are doing now. But if your in views you can see the DBquery generate and use it on PMA to delete a greater number of users with theirs comments.

Be carefull, it's to do if you know what you are doing. I suggest to look at the query of view  because Drupal create a table for each CCK field so it's hell to figure the inner joins. If you have a doubt, make a save of your database before.