I was thrown into the same situation by a recent interaction with customers. They informed that there is a user account that gets re-added to security roles. The main issue which they reported was that the account which was getting added was for a user who left the company. Here are the database names where it was getting added:
SharedServices_Search_SSPDB, SharedServices_SSPDB, SharePoint_Config
I captured a profile to find how its getting added. I found that this was being done by SharePoint service at scheduled time. I checked the internet and couldn’t find anything. Most likely, this user has used his own account as a setup account and used to configure SharePoint Shared Service Provider (SSP).
I am not sure if there is a way to resolve the automatic account addition. I learned that a user should not use personal accounts for SharePoint setup. The only feasible option which I offered them to disable the account in active directory level, so it cannot be used by a user.
Reference: Pinal Dave (https://blog.sqlauthority.com)