Singleton pagetypes and blocktypes. An Attribute and InitializableModule that limits page and block types to be created only once (single instance).

Singleton.cs

public class Singleton : Attribute
{
}

SingletonTypeInitializationModule.cs

[InitializableModule]
public class SingletonTypeInitializationModule : IInitializableModule
{
    public void Initialize(InitializationEngine context)
    {
        var contentEvents = ServiceLocator.Current.GetInstance<IContentEvents>();

        contentEvents.DeletingContent += ContentEvents_DeletingContent;
        contentEvents.CreatedContent += ContentEvents_CreatedContent;
    }

    public void Preload(string[] parameters)
    {
    }

    public void Uninitialize(InitializationEngine context)
    {
        var contentEvents = ServiceLocator.Current.GetInstance<IContentEvents>();

        contentEvents.DeletingContent -= ContentEvents_DeletingContent;
        contentEvents.CreatedContent -= ContentEvents_CreatedContent;
    }

    void ContentEvents_CreatedContent(object sender, ContentEventArgs e)
    {
        var repository = ServiceLocator.Current.GetInstance<IContentTypeRepository>();

        if (repository != null)
        {
            var contentType = repository.Load(e.Content.ContentTypeID);

            var attributes = Attribute.GetCustomAttributes(contentType.ModelType);
            var isSingleton = attributes.Any(x => x is Singleton);

            if (isSingleton)
            {
                var writableContentType = (ContentType)contentType.CreateWritableClone();
                writableContentType.IsAvailable = false;
                repository.Save(writableContentType);
            }

        }
    }

    private void ContentEvents_DeletingContent(object sender, DeleteContentEventArgs e)
    {
        var list = new List<ContentReference>();
        list.AddRange(e.DeletedDescendents);

        if (!e.ContentLink.Equals(ContentReference.WasteBasket))
        {
            list.Add(e.ContentLink);
        }

        DeletingContentAction(list);
    }

    private void DeletingContentAction(IEnumerable<ContentReference> contentReferenceList)
    {
        var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
        var contentTypeRepository = ServiceLocator.Current.GetInstance<IContentTypeRepository>();

        foreach (var cr in contentReferenceList)
        {
            contentRepository.TryGet<IContent>(cr, out var content);
            if (content != null)
            {
                var contentType = contentTypeRepository.Load(content.ContentTypeID);
                var attributes = Attribute.GetCustomAttributes(contentType.ModelType);

                var isSingleton = attributes.Any(x => x is Singleton);

                if (isSingleton)
                {
                    var writableContentType = (ContentType)contentType.CreateWritableClone();
                    writableContentType.IsAvailable = true;
                    contentTypeRepository.Save(writableContentType);
                }
            }
        }
    }
}