How to remove preview button from subpanel

What you need is to set 

'css_class' => 'disabled'

for the preview rowactions on the Contacts module's subpanel

If you have customized that subpanel from studio you should have this file already in your system:

custom/modules/Contacts/clients/base/views/subpanel-for-accounts/subpanel-for-accounts.php 

if not you can create it by making a small modification via studio (like removing or adding a column to the subpanel in question) or copying the original to your custom directory. 

modules/Contacts/clients/base/views/subpanel-for-accounts/subpanel-for-accounts.php 

your viewdef will most likely include only the panels, not the rowactions, that's because it is using the default rowactions from

clients/base/views/subpanel-list/subpanel-list.php

which looks something like:

    'rowactions' => array(
        'actions' => array(
            array(
                'type' => 'rowaction',
                'css_class' => 'btn',
                'tooltip' => 'LBL_PREVIEW',
                'event' => 'list:preview:fire',
                'icon' => 'fa-eye',
                'acl_action' => 'view',
            ),
            array(
                'type' => 'rowaction',
                'name' => 'edit_button',
                'icon' => 'fa-pencil',
                'label' => 'LBL_EDIT_BUTTON',
                'event' => 'list:editrow:fire',
                'acl_action' => 'edit',
            ),
            array(
                'type' => 'unlink-action',
                'name' => 'unlink_button',
                'icon' => 'fa-chain-broken',
                'label' => 'LBL_UNLINK_BUTTON',
            ),
        ),
    ),

If you copied clients/base/views/subpanel-list/subpanel-list.php to your custom directory custom/modules/Contacts/clients/base/views/subpanel-list/subpanel-list.php and modified that, the modifications would apply to all of the default Contacts subpanels.

By overriding the rowactions in the specific subpanel-for-accounts you are modifying the rowactions only on the Contacts subpanel under Accounts.

So your custom/modules/Contacts/clients/base/views/subpanel-for-accounts/subpanel-for-accounts.php

will look something like:

$viewdefs['Contacts']['base']['view']['subpanel-for-accounts'] = array(
  'type' => 'subpanel-list',
  'panels' =>
  array(
    array(
      'name' => 'panel_header',
      'label' => 'LBL_PANEL_1',
      'fields' =>
      array(
        array(
            'name' => 'name',
            'type' => 'fullname',
            'fields' => array(
                'salutation',
                'first_name',
                'last_name',
            ),
            'link' => true,
          'label' => 'LBL_LIST_NAME',
          'enabled' => true,
          'default' => true,
        ),
        array(
          'name' => 'primary_address_city',
          'label' => 'LBL_LIST_CITY',
          'enabled' => true,
          'default' => true,
        ),
        array(
          'name' => 'primary_address_state',
          'label' => 'LBL_LIST_STATE',
          'enabled' => true,
          'default' => true,
        ),
        array(
          'name' => 'email',
          'label' => 'LBL_LIST_EMAIL',
          'enabled' => true,
          'default' => true,
        ),
        array(
          'name' => 'phone_work',
          'label' => 'LBL_LIST_PHONE',
          'enabled' => true,
          'default' => true,
        ),
      ),
    ),
  ),
)
  'rowactions' =>
  array (
    'actions' =>
    array (
      0 =>
      array (
        'type' => 'rowaction',
        'css_class' => 'btn',
        'tooltip' => 'LBL_PREVIEW',
        'event' => 'list:preview:fire',
        'icon' => 'fa-eye',
        'acl_action' => 'view',
        'allow_bwc' => false,
        'css_class' => 'disabled'
      ),
      1 =>
      array (
        'type' => 'rowaction',
        'name' => 'edit_button',
        'icon' => 'fa-pencil',
        'label' => 'LBL_EDIT_BUTTON',
        'event' => 'list:editrow:fire',
        'acl_action' => 'edit',
        'allow_bwc' => true,
      ),
      2 =>
      array (
        'type' => 'rowaction',
        'name' => 'set_hq_addrss',
        'icon' => 'fa-ok',
        'label' => 'LBL_SET_HQ_ADDRESS',
        'event' => 'list:sethqaddress:fire',
        'acl_action' => 'edit',
        'allow_bwc' => false,
      ),
    ),
  ),
  'panels' =>

This code is not tested, so check the syntax :)

Francesca