@colego welcome to the forum.
I do not use this feature but I have had a look to see how the profile can be deleted. I could not find a way to do it using Joplin itself so it seems that the profile has to be manually deleted from Joplin's configuration files.
When dealing with configuration files I would always recommend making a backup of your Joplin data just in case something goes wrong.
These files are stored at ~/.config/joplin-desktop on Linux systems and C:\users\<username>\.config\joplin-desktop on Windows systems.
In that folder you will see a folder named profile-xxxxxxxx where xxxxxxxx is a random set of letters and numbers. This is the data folder for the profile you created.
Fully shut down Joplin (File>Quit or CTRL+Q), then delete the profile-xxxxxxxx folder and the file profiles.json. Do not delete or edit any other file in the joplin-desktop folder. Restart Joplin.
