How to delete a project in InteliJ?

You can delete it from disk using your favorite file manager.

In IntelliJ IDEA the project can be removed from the recent projects list using the Del button on the selection on the Welcome Screen (this doesn't remove the physical project files).

By "physical project files" do you mean the files at the project location folder including the project location folder?.The Welcome Screen comes when you start IntelliJ and can it be made to come up after IntelliJ has opened?

The way you are suggesting is to go to the project location and delete the project folder, isn't it?

Yes, nothing will be removed from disk when you delete the project from the welcome screen. It's only removed from the Recent Projects list.

Welcome screen is available when no projects are open. If some project opens automatically on start, you can close it in File | Close Project.

You can also use File | Open Recent | Manage Projects...

In Windows 10, the project highlighted, Right click on the project|Show in Explorer|Delete the Project Folder.IntelliJ discards the project from the Project Window when the File Explorer is closed But when the Task View button is clicked to select another window IntelliJ recreates the project folder and the project is shown in the Project Window.

How to deal with this?

You have to close the project in IntelliJ IDEA before deleting the files from disk. If the project is open, it's state is loaded in memory. When you delete the files and close the project, IDE saves the project again into the same directory.

Is there a way to delete only the IDEA project (meaning .idea and .iml files), but without deleting the source?
I would like to reimport Maven project, because apparently something went wrong when importing it.

Not from the IDE, you would need to close the project and use your favorite file manager for that.