Create a list box, and instead of selecting a field, select "<Expression>". Enter if(left(name,2)='BA',name). You should then have a list box of names that start with BA.
There are other ways to handle what you want than a list box, button and bookmark, though. As one example, create a field, "Eliminate Customers With Names Starting with BA?" with value Y for customer names that DON'T start with BA, and null for customer names that DO start with BA. Or use the full name of the field instead of Y and display as a checkbox. If they select Y (or checkmark the field), all customers with names starting with BA will be excluded.
Generally speaking, I try to work with QlikView's normal associative logic and normal interface rather than adding buttons, macros, bookmarks, and so on. I figure that having a consistent, generic QlikView interface for each application will pay off in simplified user training. And it can be simpler for maintenance as well. If all of your applications are filled with buttons, of course, your users will be used to that, and that could then be the better approach in your case.