When the Customize dialog box is open in ArcMap, you can right-click on a Command to set its appearance to Text Only. Instructions provided are to programmatically set the appearance to Text Only for a custom tool, or command button that implements ICommand.
Refer to the article in Related Information
Private Property Get ICommand_Bitmap() As esriCore.OLE_HANDLE
' ICommand_Bitmap = <do not provide an icon>
Private Property Get ICommand_Caption() As String
' Add your implementation here
ICommand_Caption = "My Command Caption"