Hi Pilou - if you only have a small number of styles just select one and push the Delete button a few times - each time you push it you will be deleting a style. It only takes about 2 seconds of doing this in the case you show there to delete all the styles.

If you have a large number of styles and you want to get rid of them all, then a better way to do that is to start a totally fresh model by going to File > New.

- Michael