User group promotions don't work like that, they add to new groups and only remove if a user no longer meets the criteria.
If you set user banners so they are not stackable - untics the option at ACP >User Options > Allow Banner Stacking
For the groups that users are promoted to make sure you have set a higher display priority (do this within the group settings).
Once done, users will then show just one banner for the highest promoted group they are in.