After a project/event has already happened, you will want to Archive it so it no longer shows up in your Open Projects folder.
To do this, start by opening up the project. Once opened, navigate to More>Archive Project in the upper right hand corner.
The project will be successfully archived.
Please note: once archived, the project can still be accessed. You may still reference it, duplicate it and treat it like a normal project.
You can locate the project in the Archived Projects folder on the bottom left hand side of the page.