At this point, and in your circumstance where the pet store doesn't take fish back, there is no reliable way to now make bullies unaggressive in this same tank. For example, you could add more fish, but this may help or may make things worse and then you're stuck with an even bigger problem if you can't take them back. You could try rearranging the tank, adding hiding spots, etc., but there's no guarantee this works with fish that are persistently aggressive to another fish.
Therefore, the realistically reliable options are:
1) Separate the bullying fish, either with a divider or by putting them in their own tank, tub, bucket, etc. Having to do this is actually pretty common with some types of fish, meaning you need options to adjust things when needed. This works when either you have more than one tank to switch fish around to or you can bring problem fish back to the pet store or, less permanent, you can divide the tank. Typically, you'd expect the GT or GTs to eventually get too big for your other fish to mess with, but for now neither keeping them together nor putting one in and out of a box or bucket solves the problem.
2) trade, sell, or give the problem fish away and rethink/redo tank inhabitants.
Unless you're dealing with peaceful schooling or community fish, it's just a fact of life that fish you put together don't always work and you have to have options for separating them or making changes, as opposed to taking them in and out of buckets or boxes.