Hi Shigeru, 

you cannot delete the demo data once inserted into the database.

However, the developers can remove the data by inserting the query. But, I am not suggesting this way coz this is not the proper way to wipe out the data. If you do this, you will face that when you install/upgrade a module, the module has come with demo data too.

Therefore, I suggest creating a new instance without demo data load instead of deleting these entries will be far more suitable for anyone.

If you have some data in this database that you don't want to enter manually in the new database, then I suggest following the below steps.
First, create a new instance.
Second, Install all the apps that were installed in the old one. (Go to apps and install all the apps)
Third, in the old instance, you can export the entries in CSV/XIs formates.
Forth, now remove the demo data entries and import that file to the new instance.

let me know If you need more help from me.