On Mastodon, you can use a follow request system to restrict who can follow you. When it’s switched on, no one can follow you unless you manually approve their request.
To restrict who can follow you, log in through the website and go to Edit profile > Require follow requests, tick the box and click Save changes. After you’ve done this, a padlock icon 🔒 will appear next to your username on your profile.
Note that if you are screening followers, don’t screen people out just because they have blank profile pictures, as many blind users don’t use profile pictures. The best way to screen potential followers is to read what they have written about themselves and what they have posted.