...
The easiest way to remove data is to use the Manage Forms feature. This is available to projects with a Pro software plan or higher. You can use it to archive all of your project's data. You can also use the Manage Forms tool to locate a specific subset of users and archive their form submissions.
Archive Single Forms
If you just need to remove select To remove specific data, you can locate individual forms using the Submit History report and archive them as described on Archive Forms. This is available to all users; however it may be difficult to use with a large volume of forms.
...
The absolute definitive way to get rid of all data is to use a separate project space for practice and testing. If you have lots of data that you want to get rid of, this may be the best option; however it will require to set up everything in the project space again.
Picking the Right Tool
In general, if you want to get rid of a large set of project data, then the Manage Forms (Archive) feature is going to be the most helpful. However, if you want to target specific types of data, here is a chart which indicates the best tool for each goal:
...