I was wondering what was the correct way of stopping a person that wants to join you for prayer but you where actually praying a Sunnah prayer. Second is how would a person know if the person is praying a Sunnah or Fardh prayer.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is permissible for a person who wants to pray an obligatory prayer to be led in prayer by a person who is performing a supererogatory prayer according to the most preponderant opinion of the scholars. The evidence is that Mu’aath used to pray (the obligatory prayer) with the Prophet and when he goes back to his people, he used to lead them in performing the same prayer. [Al-Bukhari and Muslim] This means that Mu'aath performed a supererogatory prayer with his people while they performed an obligatory prayer. Therefore, there is no need to prevent someone who wants to perform the prayer with you because you are performing a supererogatory prayer, and there is no need for him to know whether you are performing a supererogatory prayer or an obligatory one.   

Allaah Knows best.