Hello Tah,
I had a problem with this as well. It seems like there's a communication problem between Kobo and Kobocat. When the project is deleted in Kobo it seems that it is not deleted in Kobocat which I think is the origin for the information that shows up in Kobo. Whatever the reason, the following worked for me:
1. From the KoboToolbox dashboard select, "Projects (Legacy)"
2. Select the project you'd like to delete
3. Select,
"Project Settings" 4.
Delete project with all data and its form.Regards,
Jake