This MODX plugin hides certain Content Blocks buttons from a specific MODX user group. It also disables the drag and drop functionality, so that you can provide some users with a more locked down version of Content Blocks.

cbContentEditor.php

<?php
/**
 * Content Blocks cbContentEditor Plugin
 * 
 * This plugin hides certain CB buttons from certain MODX user groups
 * and disables the drag and drop functionality, so that you can provide
 * some clients with a more locked down version of CB.
 * 
 * I would suggest setting up CB templates and defaults in the CB component manager,
 * so that predefined layouts are loaded automatically when a new resource is created. 
 * It is also recommended that you hide the main content area on "resource/create" using form 
 * customisation, other wise when a user creates a new page, they will be able to delete blocks 
 * and use drag & drop, because no Resource ID has been defined yet (this only happens on save).
 * 
 * Tested on: MODX 2.6.1, Content Blocks
 *
 * Events: OnDocFormPrerender
 *
 * @author Jon Leverrier <jon@youandmedigital.com>
 *
 */
switch ($modx->event->name) {
    // load on system event OnDocFormPrerender
    case 'OnDocFormPrerender':
        
        // get the current resource ID
        $resource = $modx->resource;
        // for newly created resources, there will be no ID until the page is saved. So if there
        // is no ID, stop here...
        if (!$resource) {
            return;
        }
        // check to see if CB is enabled on a resource
        $isContentBlocks = $resource->getProperty('_isContentBlocks', 'contentblocks', null);
        
        // only load for a specific user group and if CB is enabled on the resource
        if ($modx->user->isMember('Client Web Editor') && $isContentBlocks = true) {
                // load custom script to prevent drag and drop feature in CB
                // (need to find a better way todo this)
                $modx->regClientStartupHTMLBlock('
            	   <script>
                        jQuery(function($) {
                            var checkCB = setInterval(function() {
                                if (ContentBlocks.initialized) {
                                    $("#contentblocks .contentblocks-field-wrap").addClass("prevent-drag");
                                        clearInterval(checkCB);
                                    }
                                }, 50);
                        	});
                    </script>
                ');
                // remove certain buttons from the CB interface
                $modx->controller->addHTML('
                    <style>
                        .contentblocks-field-delete,
                        .contentblocks-add-block,
                        .contentblocks-add-layout,
                        .contentblocks-layout-menu,
                        .contentblocks-layout-move-up,
                        .contentblocks-layout-move-down,
                        .contentblocks-add-content-here,
                        .contentblocks-layout-settings,
                        .contentblocks-field-gallery-url,
                        .contentblocks-field-upload,
                        .contentblocks-field-image-url {
                            display: none !important;
                        }
                    </style>
                ');
            
    		
        } else{
            // if the check fails, do nothing
            return;
        }
        break;
}

Hi @jonleverrier - thanks a lot for your fast reply and sorry for not answering earlier.

I just realized, that the plugin works as it should, but can't - up to now - fulfill one respectively two of my last following open wishes:

1. Option to disable the drag&drop image upload
2. Option to hide the text „or drop images here“ (unfortunately this text is not wrapped with a tag)

I will also ask for the option in this post...
https://forum.modmore.com/t/option-to-disable-the-drag-drop-image-upload-and-upload-from-url/699
... , but maybe someone has already extended this nice plugin or has a useful hint?