Unfortunately this is not possible any longer, all you can do is one of the following:

1, Wait for the user to return back to your room to ban them but this only lasts for a certain amount of time.

2, Go to your own list of followers and search** through the pages of your followers looking for the username then clicking at the top right ‘option menu’ this gives you ‘Block’ & ‘Delete’ options.

Block option: User will no longer receive your follower notifications. The follow button in your room will be disabled so that the user is not able to re-follow you.

Delete option: The user is removed from your follower list, but that user can re-follow you in the future.

**There is no way at the moment to search for certain usernames through your follow list.  You have to look at every single page until you find the user.