Somewhere in your code you will most likely see something like the following in the 'Optional Interfaces' section:
Public ReadOnly Property ModuleActions() As Entities.Modules.Actions.ModuleActionCollection Implements Entities.Modules.IActionable.ModuleActions
Get
Dim Actions As New Entities.Modules.Actions.ModuleActionCollection
Actions.Add(GetNextActionID, Localization.GetString(Entities.Modules.Actions.ModuleActionType.AddContent, LocalResourceFile), Entities.Modules.Actions.ModuleActionType.AddContent, "", "", EditURL(), False, Security.SecurityAccessLevel.Edit, True, False)
Return Actions
End Get
End Property
It is the iActionable interface that determines what controls show up in the ModuleActionMenu dropdown. Remove the corresponding Actions.Add and you should remove that "Add Content" control from the drop down.