The ability to hide/archive a project is something on our roadmap but isn't currently available. In the mean time you can edit the list of authorized users on the project to remove your account and that will at least get it out of your project dropdown list.
I would put in a feature request for the following:
• Delete Projects - from a high level permission only - with a double pop-up warning
• Copy content, collections, cards, or layouts from one project to another.
Seems pretty clear to me these solutions are high on a few people's lists!
Currently, you cannot delete or remove projects. The ability to hide projects ("archive") is on the roadmap. This will enable you to remove projects from view that may be duplicates, used for testing or no longer active.
If what you are trying to do is remove the project from the drop down menu or from other parts of the dashboard, you can achieve almost the same thing by simply removing the users from that project. Those users will no longer see the project anywhere in the UI. The only caveat is that the admin can not remove themselves from the project.
You still cannot change a project from one top-level to two top-level or the other way.
If you built a project with the wrong number of top-levels, you must create a new project.
You cannot currently delete the project that was built incorrectly.
And there is no method to copy content, collections, cards, or layouts from one project to another.