Application.Options.CheckGrammarAsYouType & .CheckSpellingAsYouType é o que você está procurando.
Exemplo:
Sub GrammarSpellingOn()
Application.Options.CheckGrammarAsYouType = True
Application.Options.CheckSpellingAsYouType = True
End Sub
Sub GrammarSpellingOff()
Application.Options.CheckGrammarAsYouType = False
Application.Options.CheckSpellingAsYouType = False
End Sub
Para ativar / desativar usando a mesma macro, com um pop-up informando a alteração feita:
Sub GrammarSpellingOnOff()
If Application.Options.CheckGrammarAsYouType = True Or Application.Options.CheckSpellingAsYouType = True Then
Application.Options.CheckGrammarAsYouType = False
Application.Options.CheckSpellingAsYouType = False
Call MsgBox("Grammar & Spell Checking turned OFF")
Else
Application.Options.CheckGrammarAsYouType = True
Application.Options.CheckSpellingAsYouType = True
Call MsgBox("Grammar & Spell Checking turned ON")
End If
Application.ScreenRefresh 'refresh to add/remove spellchecker underlines
End Sub