Twitter no longer allows adding or removing followers in bulk. So, websites that use Twitter API to allow user to add-remove followers and fans have disabled Select All feature in order to adhere to Twitter's policies.
Tweepi is one of the most-popular and easy to use site that allows you to not only remove followers but fans too. You can select to remove followers/fans based on their activeness in using Twitter, their profile picture (whether they have changed the default 'egg' or not.) or their profile completion (bio description.).
However, Tweepi too, has disabled Select All option so you have to manually go through clicking all the check boxes and it could be very troublesome to do so if you have a large number of followers/fans. Here's how you can avoid it and instead be able to select all the check boxes at once.
What we are going to do is install an add-on designed for the same purpose - to select all check boxes in Tweepi.
After downloading the add-on, make sure to install it. Then, restart your browser.
Once you are on the page where the users are listed and you need to select the check boxes, right click and select Check all on Tweepi.
Voilà! Every user will be selected!