Mailing sending to removed users
The list of recipients is created during the mailing creation process (i.e. when you assign the groups to the mailing). This is primarily to avoid any permissioning issues when sending the mailing.

The easiest approach is to edit and save the mailing again, which will create a new set of recipients from the groups.

Alternatively if the mailing is still in scheduled stage, you could delete the relevant entries from civicrm_mailing_recipients