How can I prevent some of the supporters to be displayed in the Supporters section?

You should go at Appearance > Customize > Supporters > Blacklist and use this box to type in the usernames. Please use a comma to separate the usernames in the list (ex: username, username2, username3). Please note that if you use a blacklist, ALL supporters can be displayed EXCEPT the ones in the blacklist. If you leave this box empty, the supporters displayed will be chosen randomly from all supporters.