Delete users after they left the organization

We use Feedbear for internal project management and restrict access to the board to only those within the firm. Currently, I do not see any way to delete users once they have left the firm. Currently, the only way is to block user email addresses one by one. Additionally, I don't see any user id validation method such as an email link to validate the sign up. This would help if a user left the company but then tries to create a fake account to access the board. I guess SSO would tackle this but it would be great to manage the user list.