You can never delete a user, instead of that, you can deactivate them. To deactivate users follow these steps:
1. In Setup, enter the Quick Find box and select users.
2. Click on the edit icon next to a user’s name.
3. Deselect the already Active checkbox and then click Save.
Check this for reference: https://galvintech.com/how-to-deactivate-a-salesforce-user/