Disable globally defined Sitecore Web API filters.

ApiController.cs

/// <summary>
/// Available at url: /sitecore/api/ssc/example1/andreas/{id}/{action}
/// </summary>
[Sitecore.Services.Core.ServicesController("example1.andreas")]
[ClearSitecoreWebApiConfig]
public class MyController : System.Web.Http.ApiController
{
	[HttpGet]
	public IHttpActionResult MyAction()
	{
		// Implement
	}
}

ClearSitecoreWebApiConfigAttribute.cs

using System;
using System.Linq;
using System.Web.Http;

namespace YourNamespace
{
    /// <summary>
    /// Clear Sitecore's Web API config for a <see cref="ApiController"/>.
    /// </summary>
    /// <remarks>
    /// Removes <see cref="System.Web.Http.Filters.IFilter"/> and <see cref="System.Net.Http.Formatting.MediaTypeFormatter"/> set by Sitecore.
    /// Note that this means Authorization Filters and Security Policies defined by Sitecore.Services.Client will no longer work for this controller.
    /// </remarks>
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
    public class ClearSitecoreWebApiConfigAttribute : Attribute, System.Web.Http.Controllers.IControllerConfiguration
    {
        public void Initialize(System.Web.Http.Controllers.HttpControllerSettings controllerSettings, System.Web.Http.Controllers.HttpControllerDescriptor controllerDescriptor)
        {
            // Remove Sitecore's Sitecore.Services.Infrastructure.Web.Http.Formatting.BrowserJsonFormatter configured in
            // Sitecore.Services.Infrastructure.Sitecore.ApplicationContainer.ResolveServicesWebApiConfiguration().
            var sitecoreFormatter = controllerSettings.Formatters.FirstOrDefault(formatter => formatter.GetType() == typeof(Sitecore.Services.Infrastructure.Web.Http.Formatting.BrowserJsonFormatter));
            if (sitecoreFormatter != null) controllerSettings.Formatters.Remove(sitecoreFormatter);

            // Remove the ConfigurationFilterProvider as this is responsible for providing the globally configured Sitecore filters
            var filterProviders = controllerSettings.Services.GetFilterProviders();
            var configurationFilterProvider = filterProviders.FirstOrDefault(provider => provider.GetType() == typeof(System.Web.Http.Filters.ConfigurationFilterProvider));
            if (configurationFilterProvider != null)
            {
                controllerSettings.Services.Remove(typeof(System.Web.Http.Filters.IFilterProvider), configurationFilterProvider);
            }
        }
    }
}