You can try to make your own attribute and pass configuration to the dojo editor. This is a sample that I haven't tested:

using System;
using System.Web.Mvc;
using EPiServer.Shell.ObjectEditing;

namespace Alloy.ObjectEditing
{
    public class CustomAutoSuggestSelectionAttribute : AutoSuggestSelectionAttribute, IMetadataAware
    {
        public CustomAutoSuggestSelectionAttribute(Type selectionFactoryType) : base(selectionFactoryType)
        {
        }

        public new void OnMetadataCreated(ModelMetadata metadata)
        {
            base.OnMetadataCreated(metadata);

            var extendedMetadata = metadata as ExtendedMetadata;

            if (extendedMetadata == null)
            {
                return;
            }

            extendedMetadata.EditorConfiguration["autoComplete"] = false;
        }
    }
}

Then you can decorate your property with this attribute instead:

[CustomAutoSuggestSelection(typeof(MySelectionQueryFactory))
public virtual string MyProperty { get; set; }

Hi Mattias,

That did exactly what I needed, thanks for your help,

Mark