Hi,

You can easily do it by frontend. Purchase application have 2 user groups, One is Purchase/Administrator, Other is Purchase/User. FYI, Purchase/Administrator group already inherited with Purchase/User group. Alright then when you want to hide/disable the delete option from the action from the list and form view, follow the below step

Enable the developer mode, then go to Settings ---> Technical ---> Models. Search purchase.order model. then you can get the model record, open it and under the access right tab you can see all assigned user groups. By default the delete access checkbox have enabled for Purchase/Administrator & Purchase/User , so you have to untick the delete boolean from the needed group. then Save and go back to purchase. you can see the change

The same method , you can try from backend too by inheriting the fundamentals
Hope it works for you