Ribbon buttons cannot be hidden using HideCustomAction, at all. At least, I've tried this and it doesn't work.
You should use CustomAction for this purpose, with empty CommandUIDefinition elements, like this:
<CommandUIDefinition Location="Ribbon.EditingTools.CPEditTab.Styles" />
This approach was tested many times and works like a charm.
But in your particular situation, this method won't work. The point here is that
So, you can easily hide the whole
Manage Workflows SplitButton, passing "Ribbon.List.Settings.ManageWorkflows" to the CommandUIDefinition element location. But to hide one of menu items, you ought to do a lot of work.
Actually, you should replace OOTB
Manage Workflows SplitButton with your own, and write your own script, which returns the whole dynamic menu XML code.
Or, you can choose to refuse dynamic functionality, and deal with static menu approach - actually it is much easier and I recommend you to start with it, although I have the ready-to-use solution for dynamic menu.
The whole code:
<CustomAction Id="RemoveAddWorkflowMenuItem" Location="CommandUI.Ribbon">
Image16by16="/_layouts/$Resources:core,Language;/images/formatmap16x16.png" Image16by16Top="-112" Image16by16Left="-112"
Image32by32="/_layouts/$Resources:core,Language;/images/formatmap32x32.png" Image32by32Top="-160" Image32by32Left="-416"
<MenuSection DisplayMode="Menu" Id="Ribbon.List.Settings.ManageWorkflows.Menu.Manage">
<Button Id="Ribbon.List.Settings.ManageWorkflows.Menu.Manage.EditSettings" Command="EditWorkflowSettings" MenuItemId="Ribbon.List.Settings.ManageWorkflows.Menu.Manage.WorkflowSettings.Menu" LabelText="Workflow Settings"/>
<MenuSection DisplayMode="Menu" Id="Ribbon.List.Settings.ManageWorkflows.Menu.CreateInSP">
<Button Id="Ribbon.List.Settings.ManageWorkflows.Menu.CreateInSP.CreateWorkflowInDesigner" Command="CreateWorkflowInDesigner" LabelText="Create a Workflow in SharePoint Designer"/>
<Button Id="Ribbon.List.Settings.ManageWorkflows.Menu.CreateInSP.CreateReusableWorkflowInDesigner" Command="CreateReusableWorkflowInDesigner" LabelText="Create a Reusable Workflow in SharePoint Designer"/>
Screenshot of the customized splitbutton:
P.S. If you often work with SharePoint Ribbon, I would recommend you to check out my open source project SharePoint 2010 Fluent Ribbon API, which simplifies ribbon creation and customization. It is completely free and licensed under Ms-PL.