Remove the default delete action in Django admin

In your admin class, define has_delete_permission to return False:

class YourModelAdmin(admin.ModelAdmin):
    ...

    def has_delete_permission(self, request, obj=None):
        return False

Then, it will not show delete button, and will not allow you to delete objects in admin interface.

If you want to remove all the action:

class UserAdmin(admin.ModelAdmin):
    model = User
    actions = None

If you want some specific action:

class UserAdmin(admin.ModelAdmin):
    model = User
    actions = ['name_of_action_you_want_to_keep']

You can disable “delete selected” action site-wide:

from django.contrib.admin import site
site.disable_action('delete_selected')

When you need to include this action, add 'delete_selected' to the action list:

actions = ['delete_selected']

Documentation

If you are using that model, as a foreign key in some other model.
Then by using PROTECT constraint for that foreign key you can disable deletion for that model in Django admin.

For Example,

class Exam(models.Model):
    student = models.ForeignKey(User, on_delete=models.PROTECT)
    marks = models.IntegerField(default=0)

By adding PROTECT constraint to User model through the foreign key present in Exam model, I have disabled the power (in Django admin or elsewhere) to delete students (User) who have written exams.

Giving credit to @DawnTCherian, @tschale and @falsetru

I used:

class YourModelAdmin(admin.ModelAdmin):
...
  def get_actions(self, request):
      actions = super(YourModelAdmin, self).get_actions(request)
      try:
          del actions['delete_selected']
      except KeyError:
        pass
      return actions

  def has_delete_permission(self, request, obj=None):
      return False

It removes the delete action from the list view and the delete option from the detail view.

You can globally disable bulk delete action and enable for selected models only.

Documentation from django website

# Globally disable delete selected
admin.site.disable_action('delete_selected')

# This ModelAdmin will not have delete_selected available
class SomeModelAdmin(admin.ModelAdmin):
    actions = ['some_other_action']
    ...

# This one will
class AnotherModelAdmin(admin.ModelAdmin):
    actions = ['delete_selected', 'a_third_action']
    ...