How to remove dbcreator server role?
You can only add logins to server roles, not database users. To remove a login, you use ALTER SERVER ROLE ... DROP MEMBER:
ALTER SERVER ROLE dbcreator DROP MEMBER login_name;

Please stop using the stored procedures; they're all deprecated. For example, from the sp_addsrvrolemember documentation:

This feature is in maintenance mode and may be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. Use ALTER SERVER ROLE instead.