Looking into source code of project, this definitely was not implemented. So the best option is to encrypt zip file as suggested above.
# Encrypt the package into a CFB using the password provided
# This is not ready yet
def encrypt(file_name, password)
return false
# moc = MsOffCrypto.new(file_name, password)
# moc.save
end
Also I did investigation about vba scripting, and this is available by their macro
Dim password As Variant
password = Application.InputBox("Enter Password", "Password Protected")
Select Case password
Case Is = False
'do nothing
Case Is = "easy"
Range("A1").Value = "This is secret code"
Case Else
MsgBox "Incorrect Password"
End Select
But unfortunately I don't see that it can be possible to execute vba scripts passing it some function of caxlsx gem