Remove all buttons from a EPiServer CMS 7 XHTML / TinyMCE editor and add only the buttons you want to use.

EditorButtonEditorDescriptor.cs

using EPiServer.Core;
using EPiServer.Editor.TinyMCE;
using EPiServer.Shell.ObjectEditing;
using EPiServer.Shell.ObjectEditing.EditorDescriptors;
using Site.EPiServerAdmin.Attributes;
using System;
using System.Collections.Generic;
using System.Linq;
 
namespace Site.EPiServerAdmin.EditorDescriptors
    [EditorDescriptorRegistration(TargetType = typeof(XhtmlString), EditorDescriptorBehavior = EditorDescriptorBehavior.PlaceLast)]
    public class EditorButtonEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            var attributeValue = metadata.Attributes.OfType<EditorButtonsAttribute>().SingleOrDefault();
            if (attributeValue == null) return;
 
            var model = (PropertyData) metadata.Model;
            var settings = (TinyMCESettings)(model).GetSetting(typeof(TinyMCESettings));
            settings.ToolbarRows.Clear();
            settings.ToolbarRows.Add(attributeValue.ToolbarRow);
 
            var options = new TinyMCEInitOptions(TinyMCEInitOptions.InitType.EditMode, settings, (IContent)null);
            metadata.EditorConfiguration["settings"] = options.InitOptions;
 
            base.ModifyMetadata(metadata, attributes);
        }
    }
}

EditorButtonsAttribute.cs

using EPiServer.Editor.TinyMCE;
using System;
 
namespace Site.EPiServerAdmin.Attributes
{
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    public class EditorButtonsAttribute : Attribute
    {
        private ToolbarRow _toolbarRow = new ToolbarRow();
 
        public EditorButtonsAttribute(params string[] toolbarRowButtons)
        {
            foreach (var toolbarRowButton in toolbarRowButtons)
            {
                _toolbarRow.Buttons.Add(toolbarRowButton);
            }
        }
 
        public ToolbarRow ToolbarRow
        {
            get { return _toolbarRow; } 
            set { _toolbarRow = value; }
        }
    }
}

TestBlockModel.cs

using EPiServer.Core;
using EPiServer.DataAnnotations;
using EPiServer.Shell.ObjectEditing;
using Site.EPiServerAdmin.Attributes;
using Site.EPiServerAdmin.Constants;
using System.ComponentModel.DataAnnotations;
 
namespace Site.Models.Blocks
{
    [DefaultContentType(
            DisplayName = "Testblock",
            Description = "A block used to test things.",
            GUID = "226F3E63-4DB5-4546-AFFA-B9C4A9C6BAC6")]
    [DefaultImageUrl]
    public class TestBlockModel:BlockData
    {
        [EditorButtons(new[] { "bold" })]
        [Display(Name = "Main contents", GroupName = GroupNames.Default)]
        public virtual XhtmlString MainBody { get; set; }
    }
}

Doesn't appear to work with newer versions of EPiserver (CMS 10+). It just rendered an empty readonly text area for me.