Currently, the presets of the models and hairstyles that you have created cannot be deleted directly on VRoid Studio.
You can find the preset files in the following folder on your PC, and delete them directly.
The vroid file of the model you created
Delete the vroid file in the "avatars" section below.
In Finder, choose Go ＞Home＞ vroid\avatars
Delete the vroid file in "hair_presets" below.
("AppData" is a hidden folder and can be shown by checking the "Show hidden files, folders, and drives" checkbox in the folder options.)
(Library is hidden by default, so you can open /Users/<username>/ and press "command+shift+.)
※In the Finder on your Mac, choose Go > Go to Folder.Type the folder’s pathname.