To disable that behabiour you've to use ActivityDesignerOptionsAttribute, in particular its AllowDrillIn property.
Use it on your activity class:
[ActivityDesignerOptions(AllowDrillIn = false)]
public sealed class MyActivity : CodedActivity
{
/* ... */
}
Or if you're using IRegisterMetadata:
internal class Metadata : IRegisterMetadata
{
private AttributeTable attributes;
// Called by the designer to register any design-time metadata.
public void Register()
{
var builder = new AttributeTableBuilder();
builder.AddCustomAttributes(
typeof(MyActivity),
new ActivityDesignerOptionsAttribute{ AllowDrillIn = false });
MetadataStore.AddAttributeTable(builder.CreateTable());
}
}